Should the case wind up in the Court of Final Appeal, it may come just in time for Chief Justice Andrew Li Kwok-nang to voice a final opinion on Hong Kong's unique position under the 'one country, two systems' formula before he retires next September.

The starting point for this case was prosaic. Some lawmakers were dismissive when they began receiving legal letters from developer New World China Land challenging their right to summon witnesses in an investigation into its post-retirement employment of Leung, a former director of housing.

Leung resigned within days of taking up his post amid allegations of a conflict of interest. Legislators saw the legal challenge as a tactic to stop the company's chairman, Henry Cheng Kar-shun, testifying a second time. After all, in April, in his first appearance, Cheng had shown his discontent at having to do so.

New World applied for the judicial review in July. Two weeks later, the stakes in the case were raised when local delegates to the Chinese People's Political Consultative Conference complained to a top Beijing official about Legco increasing its appetite for power. Speaking after the meeting with Du Qinglin , director of the Communist Party's United Front Work Department - responsible for national unity - Chan Wing-kee said many of his colleagues had raised concerns over 'a legislature seeking increasing power for itself'.

A member of Cheng's legal team acknowledged the public had initially written them off. 'People were saying, 'You cannot be serious' when we first offered our arguments, but I hope now they realise we were asking some very serious questions with evidential support,' said the lawyer.

That the case was more than a delaying tactic became clear when legal arguments began last month.

Representing Cheng was Dinah Rose QC, considered a rising star of the English Bar (she was named the 2009 barrister of the year). She was supported by Sir John Swaine, the city's most senior barrister. Another QC, Lord Lester, represented the lawmakers. Martin Lee Chu-ming, second in seniority to Swaine and a former Basic Law drafter, represented lawmaker Leung Kwok-hung, and another senior counsel, Michael Thomas - the attorney general during the signing of the 1984 Sino-British Joint Declaration - represented the justice secretary, Wong Yan-lung.

Wong is understood to have taken a strong personal interest in the case. Thomas told the court it was important the holder of that office maintain 'the integrity of the constitution of Hong Kong, to see it properly interpreted, properly applied and assist the court to that effect'.

The case centred on the meaning of Article 73 of the Basic Law, which says 'the legislative council of Hong Kong' shall exercise powers including that to 'summon, as required ... persons concerned to testify or give evidence'. Was this power exercisable only by Legco as a whole or could it be used by committees?

Rose submitted that it was clear from the context of the Basic Law that the omission of any reference to committees meant that only the full council, sitting with a quorum of at least half the legislators, could exercise such 'coercive powers'.

If the court should find any ambiguity in the wording of the constitution, Rose argued, it should consider documents created during the drafting of the Basic Law in the 1980s and other records from that time. These, she said, showed 'a clear policy choice was made' to state only the full council should exercise such powers.

Several drafts of the Basic Law were prepared before the final version was adopted by the National People's Congress in 1990.

In the first draft, published for public consultation in April 1988, the equivalent to what is now Article 73 did not confer on Legco the power to summon witnesses. But Lee and another pan-democrat, Szeto Wah, said Legco should have the power to investigate and receive evidence to better enable it to monitor the government. They suggested amending the draft to specify that Legco 'and its subordinate committees shall have power to summon any person to appear before them to testify and produce documents in his possession'.

The second draft published in 1989 included this power. But the Law as adopted does not expressly say committees can exercise it. Rose argued that the fact drafters did not choose Lee and Szeto's wording was an indication they did not agree subordinate committees should have such a power.

Her arguments raise the question of how judges should go about deciding what a law means. Should they restrict themselves to its wording or look at other documents which might give a clue as to the real intentions of those who drafted it?

Common law courts have traditionally been reluctant to go much beyond the wording of the law. However, the National People's Congress Standing Committee, which has the final say on the Basic Law's meaning, attaches great significance to drafters' intentions.

The chief justice provided the city's courts with guidance on how to interpret the Basic Law when ruling on the landmark right-of-abode case in 1999. It was a living instrument to 'implement the unique principle of 'one country, two systems'' but one intended to meet changing needs and circumstances, he said. The courts should avoid a rigid interpretation and should consider the context of a provision by referring to the Basic Law itself and 'extrinsic' materials such as the Joint Declaration.

The starting point for the courts remains the words of the Basic Law itself. Ruling in another right-of-abode case in 2001, the chief justice said the courts' role was to 'ascertain the legislative intent as expressed in the language. Their task is not to ascertain the intent of the lawmaker on its own'.

Still, the city's top court has accepted that judges can consider documents which do not form part of the Law. So which documents?

Simon Young, director of the Centre for Comparative and Public Law at the University of Hong Kong, said that while the city's courts try to avoid using materials relating to the legislative process, the NPC Standing Committee treats such documents as a primary source.

He said the committee and, at times, the Department of Justice under the administration of former chief executive Tung Chee-hwa, had cited 'original legislative intent' as the basis for interpretation. The government had also sought to rely on the opinions of the Beijing-appointed Preparatory Committee in 1996 as evidence of what drafters of the Basic Law intended. (Critics argue these opinions were expressed years after the Basic Law was drafted and so are of little value.)

The Court of Final Appeal's reliance on a method of interpretation which differs from the NPC Standing Committee's has been seen by many as having helped uphold the 'one country, two systems' principle. Lawyers observing the New World hearing were delighted to hear Thomas, the counsel for the Secretary for Justice, speak so emphatically in support of principles set down by Li, rather than the committee.

Mainland legal scholars who took part in the Law's drafting have often said what they believe it means.

In the foreword to a book on its drafting, Professor Xiao Weiyun said the Law emphasises information and materials discussed during its drafting 'to explain the legislative intentions of the drafters'. Xiao was a convenor of the drafting committee's subgroup on the political system.

In last month's hearing, lawyers for the lawmakers cautioned the judge against relying on materials which purport to show what the drafters intended, citing the concern expressed by Young that only some of the drafting materials have been made public and that therefore they may not present the full picture.

When Cheung hands down his judgment today, he will not only be deciding on the nature and extent of Legco's powers but also the extent to which he should rely on Basic Law drafting materials. It will offer fresh insight into judicial thinking on 'one country, two systems' - and the way in which judges work out what the city's constitution really means.

But the story does not end there. The lawyers involved expect the case to reach the Court of Final Appeal.

When the chief justice announced three weeks ago that he would retire early, he said: 'The two systems are different, that's why we are 'one country, two systems'. How I've managed the new constitutional order is not for me to judge. This is a matter for the legal community, and the community as a whole to judge.'

If he gets the opportunity to head the panel of judges which will have the final word on this legal challenge, the community will have further evidence to help it judge how well Li has steered the ship between common law principles and the legal tenets of Hong Kong's sovereign.