I don’t enjoy mechanics in the dark, but I don’t enjoy walking (With flat tyred bike) on my tod for 7 miles in the dark/mud more
“so will replace the cassette I think”
How much use has the ‘new’ chain had? It will have been ‘trying’ to ‘wear’ to the worn casette. If you use a new casette with your ‘new(er)’ chain the casette will have to ‘wear’ to the ‘new'(er) chain?
I always buy chain/casette/chainrings all together. Then not wasting anything new by making it ‘wear’ to anything not new
Last couple of times I’ve swapped to a new chain at or just (I mean just) over 0.75% stretch (on the 1st chain), again nearing (or at) 1% stretch on the 2nd chain aand then leave the 3rd chain on until everything is dead and slipping/snapping
Going by waht I’ve read on here the best thing to do would be to have 3 chains from new and alternate them every or evry other ride. (cleaning everything properly between obviously). I can’t be bothered with this level of drivetrain dedication/faff