A PRIME military site in Queensway currently housing British soldiers has emerged as the main obstacle to a Sino-British agreement on defence land. China wants to keep the site for use as married quarters for the People's Liberation Army (PLA) while Britain wants to sell it for redevelopment. Queens Lines is next to Colvin House, the site of the future British consulate. It is understood to have been identified by the British side as a valuable site for redevelopment for either commercial or residential use. Comprising two towers and housing more than 40 families of the British garrison, Queens Lines, a part of the former Victoria Barracks, would be expected to sell for hundreds of millions of dollars given the lack of prime sites in the central business district. The disagreement over the fate of the barracks is one of the few outstanding obstacles to a full agreement on the handover of defence facilities. Another remaining difference is Britain's demand for a written commitment from China for the PLA to return any unused military sites to the Special Administrative Region (SAR) Government after 1997. This is to ensure that any revenue generated by disposing of the sites will go to the Hong Kong Treasury. British and Chinese officials in the Joint Liaison Group (JLG) have been working hard in the last few months to bridge the differences but given the current hostility between the two nations over constitutional issues, neither side is optimistic that an accord can be reached at this week's plenum. Neither the British nor the Chinese JLG teams are expecting agreements on other major subjects, including the Container Terminal 9 project, at the three-day plenum to be held in London. Defence matters are expected to be among the toughest issues. It is understood the British side has attempted to have major military sites in central urban areas approved for redevelopment for political and economic reasons. Politically, the British Government has argued that PLA troops to be stationed in the territory should be kept away from the city centre. Economically, the redevelopment of existing sites in the central parts of the built-up area will generate huge amounts of revenue for the Treasury. On the issue of redeveloping Queens Lines, it is understood Chinese negotiators have argued that they have already acceded to demands from the British side for the redevelopment of several other sites, and that British negotiators had tried to ''take a yard after getting an inch''. Although officials said argument over most of the sites had been settled, they admitted that coming to an arrangement on Queens Lines remained difficult. Further negotiation is also expected on ways of ensuring that military sites unused in the SAR will be handed back to the Hong Kong Government for its own purposes. This is despite the fact that the Chinese Government has promised to make a unilateral declaration to that effect. The Chinese side has said it considers ''unreasonable'' a British demand that the commitment has to be spelled out to the British side, rather than the Hong Kong Government, at the JLG. ''We will only make a commitment to [the] Hong Kong people, not to the British Government,'' a Chinese official said.