Got a wee problem with my hardtail.
If you put a bit of power down when you pedal the drive train slips and makes a big unhappy noise.

The chain was badly stretched (lent it to a mate who didn’t look after it) so I got a new Shimano chain and cassette and put them on, but it’s still happening.

Bit of a loss as to what the problem could be. Derailleur?

Worn chainring more likely I’d say

Most likely;
Chain & rings/cassette not meshing & slipping due to wear.
Same problem recently on my bike was worn freehub paws not engaging with freehub. (SuperStar Tesla hub)