To update...Belt arrived..cleaned pulleys .put it on...squeak gone. Correct belt for my S320 was Contitech 6K2225 .
Seems the previous owner had the wrong belt on...old belt was about 3/4" longer and he had made the problem worse by using loads of belt dressing.
My car does not have the mechanical air pump (smog pump) nor have rear AC...takes longer belt if yours has these. Great to have that annoying squeal gone finally !