Thanks everyone for the replies!
I've double-checked various things but without success so far. Dirty switch contacts, board edge contacts etc I've gone over with no result, likewise the transport path - guides, rollers, lifter mech etc.
Re. testing: I fully agree with those saying that using commercial pre-rec tapes is at best approximate, but I did explain in the OP that this was a temporary measure, using recordings I know very well and use as regular test tracks and that I also as digital issues for comparison. Unfortunately the degree of flutter is so pronounced that this method suffices at present!
I also mentioned in the OP that I have a W&F meter which I'll unearth once the degree of W&F stops being so audible! That's going to have to wait until the Xmas holidays I think, when I can strip the machine down properly, give it a thorough service & see what's going on.
