Hi again, just to say that I´ve had the valves done to the specifications given in the BMW manual. The clatter and chatter has disappeared, the engine is running smoothly and quietly like a dream, and all the power is there! After 6 years (or more) with no adjustment, clearances were upto 0.70 mm !!! ...luckily without flaking, rutting or other damage to cams. My mechanic apologized profusely for having assumed all this time that they were hydraulically regulated like most of the M30´s.
Must be a good engine after all the s*** it has gone through due to so much negligence.
Thanks again everyone, for all your information and moral support!
Cheers.