So in the workstand the pedals spin when the bike is freewheeling.

I've stripped the hub and removed the freehub and cleaned inside. Problem is still there. Might one of the cartridge bearings be shot? Looks like there are 3 inside the freehub? Is it likely to be the outermost one? I cleaned inside the top one and regreased but no joy. No idea how I get these bearings out and back in the freehub though.

Planet X hub by the way

if you're anything like me it'll be the one least easy to get at 🙁

I guess you're running gears and not SS – too tight chain on SS gives me those symptoms.

if you're sure it's not sticky pawls then bearings look like the most likely cause.

MY ProII did this – turned out to be a seized / rough running bearing.

I've worked out the cause was washing the bike and the dirty water working its way into the hub and drying causing the dirt to sieze it up over time.