My drive train has started to behave oddly. Any hint of a back pedal makes a terrible clatter and the chain stays wrapped around the chain ring
I have tried
Chain rings – newish and it happens on all three
Bottom bracket spins freely
Jockey wheels spin fine
Free hub fine
Chain newish and well oiled

Any suggestions welcome, this is driving me mad

1) Mech hanger alignment
2) Wear, even though you think it isn’t
3) Don’t pedal backwards

No. 3 will fix it for certain

double post

check your b screw adjustment.

chain could of stretched a bit allowing the top jockey wheel to touch the cassette.

that can make it pretty clunky pedalling backwards.

I had this recently, one of my pawls had snapped and driven itself into the hub shell, so would catch on the freehub and stop the cassette spinning when freewheel or backpeddling,
Took a lot of working out.
If it’s not that, then no idea

What drivetrain? “Thou shall not backpedal in the 42t cog” or “small chainring with smallish cog ready for storage” on the gnarmac?

I know you said “fine” but sounds a lot like a sticky freehub

Interestingly enough I had the same as richen today on rebuilding the fatty hub/freewheel, couldn’t understand why freehub was sticking enough to start to cause chain wrap, turned out half a pawl tooth had split off and was sat at back of hub. It’s worth checking.