WHEN the remaining five helicopters of 660 Squadron fly over HMS Tamar just before Christmas, it will mark the start of a major phase in the withdrawal of the British military from Hong Kong. The British Army Air Corps unit will officially stop flying on December 31 and will disband in March next year. Advertisements have already been placed overseas offering the ageing Scout helicopters for sale ''as is'', and a number of prospective buyers have expressed interest. Next year the scaling down of the territory's military operations escalates, with three major bases handed back to the Government and three landing craft withdrawn from service. By far the most significant change in terms of manpower will be the withdrawal of the 680-strong Black Watch, the last British Army battalion posted to Hong Kong. On August 21, 450 men will leave the territory, while the rear party will depart on September 28. The amalgamation of 6 Gurkha Rifles with 2 Gurkha Rifles to form 1 Royal Gurkha Rifles, to be based at Cassino Lines, will take place on September 1, resulting in a further reduction in manpower. This is believed to form part of a wider package covering an agreement, reached after more than seven years of talks in the Sino-British Joint Liaison Group (JLG), on the handover of military sites. While military lands will feature on the agenda for this week's JLG talks, it is believed the next round will focus on a number of key areas, including the reclamation of the HMS Tamar basin, Gun Club Hill Barracks, Blackdown Camp, and residential properties. Crucial to a settlement will be an agreement for work to begin on a base for the People's Liberation Army (PLA) Navy on the south side of Stonecutters Island. This is expected to be finalised early next year. If agreement is reached, construction of the base - likely to be the biggest and most advanced naval facility in China - will start next year and be completed by July 1997. ''Next year is very significant,'' said Colonel Mike Winarick, who is in charge of organising plans for the ''draw down''. ''We hand back three of our major bases and we lose our battalion.'' A detailed plan of the ''draw down'' was devised by Colonel Winarick and is being used by the military and the Government for the initial stages of the withdrawal. He said his job was to prepare for the handover of the military estate to the Hong Kong Government and to ''get ready for the People's Liberation Army''. But one of his main areas of interest in planning the withdrawal was the welfare of the soldiers and maintaining the morale of the troops. The bases to be handed back to the Government next year are Stanley Fort, Gallipoli Lines and Perowne Barracks. Units based at the latter two sites will be relocated until their withdrawal. On April 1, 1994, 415 Maritime Troop of the Royal Logistic Corps, which controls the military's three landing craft, will disband. The Jackson, a launch used by the Commander British Forces (CBF), Major General John Foley, is to be returned to Britain next September and placed in a museum, while other craft will be disposed of. Contrary to earlier plans, the British Military Hospital is to remain in service, but with at least one floor and some sections closed, until 1997. Military spokesman Major Paddy Hartigan said the process of the draw down would not be easy. ''This is something that has been discussed and planned at length and with great thought,'' he said. ''CBF has placed great emphasis on planning.''