Thank you.

That is very useful info, especially about the derailleurs being slightly bent inwards, intentionally.

Unfortunately I don't have a record of which chain the old one was. I posted a pic above; it just says EAGLE 352. I used to buy from a shop called Chain Reaction Bikes but they went bust.

At some point, when I start having trouble with my existing stuff, I will replace the whole lot. BTW, this is the cassette I ordered
https://www.amazon.co.uk/dp/B071G9Y74V
and hopefully at that price it is not counterfeit