Hi, I've just fitted a new chain & I had a problem with it jumping,the local bike shop advised a new rear cassett,after buying a chain whip & locking tool yesterday I fitted that.But the chain still jumps but now only under load,ie,setting off at a road jnctn.the cassett I bought was the same size as I took off,the gear change seems to be working OK,

I recently had a similar problem that was due to an inflexible link in the chain. When putting the chain together, I think I pushed the two sides of the outer link together so that it wouldn't flex freely. I could easily see this when I turned the crank slowly and watched the chain go over the dérailleur pulleys; one of the links would not conform to the pulley and would not be engaged as it went around. The cure is to lubricate the link and flex it until it loosens up sufficiently.

Yep, check for a tight link, if that doesn't cure the problem, then you have a worn cassette/freewheel which has to be replaced. What happens is that as the chain, the weak link in the drivetrain (so to speak) wears, it wears the teeth on the cassette irregularly. When you put a new chain on, it doesn't mesh correctly with the worn teeth on the cogs and causes the skipping.

To avoid this problem in the future, check the chain for wear and replace well before the wear limit. I have put at least 4 new chains on the same cassette with no signs of skipping yet. Chains is cheap, cassettes, not so cheap.

Or, you can just put the old chain back on and keep riding it. It will last for a long, long time, but you run the risk of prematurely wearing out the chainrings if you go this route.

Tension adjustment on the rear cassette, should take care of it. Try tuning the barrel adjustment counter clockwise a little at a time.

Also put the chain on the small ring on the front and back and pull the cable tight.

well, I've been out on the bike today & after takeing it very easy for the first few kilomtrs it hasn't jumped all afternoon so maybe it's just sorted itself out I've done nothing to it? maybe a stiff link has workerd itself loose?or maybe the New Year Fairy took pity on me. thenks for the info.

Where do I find the tension adjuster on the "rear cassette"?

That would be your PEDALS