One of my colleagues has an elderly 6spd mtb which requires some low price TLC including a new rear wheel. What chains run on 6 spd freewheels? I'm kinda assuming that I can recommend a 7 or 8 speed chain for him – or am I wrong?

Think that is correct – the difference with 6/7/8 is with wheel dishing, and only when the jump to 9 speed came in did the chain get thinner.

yep, 6/7/8 speed common chain

9 speed different

great. thanks guys. I only started with 7spd Suntour XC-E in 1990 so 6 speed is beyond me.

Not wishing to be the bearer of bad news but I feel sure a 6 speed chain is wider than 7/8 speed which are the same.

I thought 6/7/8 were the same but even if 6 is wider, like Coleman says, just run a 8spd chain on it. Running a narrower chain is fine. I've run 9s on 8spds and 10s on 9spds loads of times without an issue.