THE senior echelons of the civil service are facing a serious lack of experience in the run-up to 1997 as the Government prepares for a new set of leaders to bring it in line with Basic Law requirements. At least 20 of the 56 most senior officers, ranging from policy secretaries to heads of department, are expected to leave the service before 1997. Either they are due to retire or their nationality does not meet the requirement of the Basic Law, barring them from taking principal official positions. The rapid thinning-out of experience in the next two years is understood to be a key concern for the Secretary for the Civil Service, Michael Sze Cho-cheung, as he plans future leadership. Senior expatriate officers, whose prospects have been blocked by locals, are said to be ready to opt for the Government's special compensation scheme and leave the service before 1997. The Civil Service Branch is understood to have given top priority to making the necessary changes now to minimise the impact later. Vacancies will be filled by accelerated promotion for many junior officers, even though such promotion might not normally be considered desirable. Although only policy secretaries with foreign nationalities are required by the Basic Law to step down, it is understood the administration is fully aware also of the possibility of an exodus of lower-level expatriate staff as their promotion prospects are affected. At least three senior expatriate staff have been told their contracts will not be renewed when they expire late next year or early in 1996. They include Deputy Secretary for Recreation and Culture Rachel Cartland, Deputy Secretary for Economic Services Elizabeth Bosher, and Deputy Secretary for Security Ken Woodhouse. The Governor's spokesman, Mike Hanson, Principal Assistant Secretary for Security Barbara Ellington, who specialises in defence matters, and senior research officer at the Political Adviser's Office, Ken Walker, are also expected to return to the British civil service before 1997. It is understood Ms Ellington will leave next year, Mr Hanson's contract expires in mid-1996 and Mr Walker will also retire in mid-1996. Others at the level immediately below policy secretary whose promotion will be blocked include Director of Administration Richard Hoare, Director-General of Trade Tony Miller, Director of Social Welfare Ian Strachan, and Postmaster-General Mike Pagliari. They are expected to leave to make room for locals to move up the hierarchy. Those at policy-secretary level who will have to go include Secretary for Health and Welfare Elizabeth Wong Chien Chi-lien, who will step down this summer, and Secretary for Works James Blake, who will leave next year. Others include Secretary for Security Alistair Asprey, Secretary for Planning, Environment and Lands Tony Eason, the Government's ''ambassador'' to North America, Barrie Wiggham, and Secretary for Financial Services Mike Cartland. Secretary for Education and Manpower Michael Leung Man-kin and Director of Regional Services Adolf Hsu Hsung turn 56 this year and are expected to retire in the next two years.