Yet another oops! I've just had another look at this bias setting thing - and now INCREASING does seem to be what's required (at least in a close up facial render). However, I'm 'almost' sure on one occasion I recall the opposite - where it was reduction of bias (in P6 AO lighting) which seemed to cure some black patches in a render for me. Now I'm wondering if this was the case or whether my bad eyes just didn't read the numbers correctly!