Me again, follow up to my own query, for anyone interested...
I picked up a "Geneva PF-740" video tape cleaner from USA (just needed to change the transformer for a 240v input model, same output). This machine scrubs the tape surface as the tape runs over brush-covered pillars - with or without the use of a cleaning fluid on the brushes (recommended by the maker for badly soiled tapes). This has worked fine for some tapes, but needed several run throughs on most (in one case 8 times, cleaning on FF and rewind = 16 passes - surprised there was anything playable left inside the videocassette after that, but there was and it was OK!). Slightly soiled tapes cleaned up fairly easily and played OK, others cleaned up but with some picture degradation (usually colour fade) or a few jitters. The worst tapes seem to be beyond hope though - presumably too much damage done already.
A couple of lessons I've learnt from this (a) videotape shelf life is limited, even in good storage conditions (unfortunately some of mine were in bad conditions!). Some references suggest 25 years maximum in good storage and with regular rewinding to air the tape and ease any uneven tension - I wouldn't trust anything for more than 10 years in optimum storage now. (b) if you've got stuff on videotape that is unique, irreplaceable or rare, get it onto DVD without delay!
Pip