I am thinking this is probably the new chain and cassette on the very old very worn chainring – any other thoughts

rear wheel bearing/hub.

You’re riding over snails?

Pedal bearings. BB bearings. Freehub. Knees.

pedals are smooth, BB is new, freehub was fine last ride before the change of chainring

I suspect you are right and it’s the chainring.

Have you checked your pockets for bags of crisps?

Been anywhere sandy? After a wet day a Chicksands my bike made crunchy noises/felt gritty when pedalling, for about a month, no matter how hard I cleaned it. I was about to replace the bottom bracket when it finally rid itself of the last few grains.

Definitely your knees then.

Was the chainring coupled with the loose cones