Okay so i put the old chain on and I wasn’t having the problem, it was working fine standing up under load, something that I wasn’t confident enough to do using the new chain because it was slipping while sat down and peddling.
The old chain is the one that came with the bike originally, a Shimano one that is now stretched to .75 on the chain tool, the 1. end still doesn’t fit in.
What is it best to do here? I’d rather buy a new cassette rather than continue to use the old chain because presumably it will wear the chainring out much quicker?
As I say I don’t know what to look for to know if the cassette is worn? Does that look like it needs replacing?
Sorry for all the novice questions, I’m really grateful for the help and advice you have all offered, thank you all very much.