I stripped and re-built my bike this week and now when I back peddle the chain goes horrendously slack at the top (chain suck style) and the chain rides up the cassette.
I thought it might be the freehub but that is moving freely, I’ve taken a link out of the chain to up the tension a bit and cleaned and oiled the chain itself.

Any thoughts?

Gear set up.

Sounds like rear mech is sticking. Try adjusting the b screw which sets the clearance between the mech and the cassette.

As above – likely following a strip/rebuild – or could be a sticky jockey wheel in the mech.

Chain routed round rear mech correctly?

Chain routed round rear mech correctly?

probably this – it’s running over one of the metal tabs.

Some times over tightening the wheel bearings can do this, usually with Shimano but I have had it with cartridge bearings with minimal adjustment.
Or chain and/or cassette could be shot, this was the cause of my last such incident. It doesn’t seem gradual, one day its fine , the next the chain rides around the sprockets