LONG-RUNNING Sino-British talks on the disposal of military land in Hong Kong are expected to take a major step forward next week. Joint Liaison Group (JLG) experts will meet for four days between February 22 and 25 in what is believed to be their longest session on the controversial subject since the talks began eight years ago. It is understood the meeting will provide the first opportunity for the experts to go over the progress made recently during technical discussions of all issues, including the size of the naval base for the People's Liberation Army (PLA) after 1997. Matters of principle are understood to have been resolved and the two sides are now able to get down to technical details on what will happen to the land now occupied by the British Garrison. Some will be taken over by the PLA and the rest will be sold for development. Next week's talks should lead the two sides closer to a final accord. British JLG team leader, Hugh Davies, said yesterday that he believed differences would be narrowed, while leader of the British defence experts' team, Alan Paul, said the two sides would be going into detailed discussions. Chinese experts are scheduled to tour the military facilities during the week. It is understood the Chinese side is dissatisfied with the size of the naval base which is to be built on Stonecutters Island to replace the HMS Tamar base in Central, which will be redeveloped as part of the Central-Wan Chai reclamation project. However, it is understood that the British believe their plans for the base meet all the requirements, including the justification officials have to put to the Legislative Council for funding for its construction. Until now, China has been reluctant to disclose the size of the PLA contingent it intends basing in Hong Kong in 1997, which will ultimately determine what will be required of its naval base. Britain is substantially scaling down the size of its garrison in Hong Kong in the run-up to 1997. In a paper to legislators, the Government said the strength of the British Garrison would be reduced from about 9,000 in the middle of last year to between 3,000 and 3,500 by early 1995. It would remain at about that level until 1997. The garrison would comprise a headquarters, an infantry battalion group, logistic support, naval patrol craft and Royal Air Force helicopters. ''A garrison of this size is adjudged to be the minimum required to fulfil its operational commitments,'' the paper said.