Paddy – if you’re still having a problem check the inner face of the QR adapter.
My Pro iii had the same symptoms as yours. Turns out the inner bearing race (in the freehub) had “eaten”/ worn into the alu of qr adapter. This means the adapter was made to squueze onto the rubber bearing seal – thus the drag and the symptoms. Off the bike it was perfect.
The replacement part from hope is a significantly different shape to the original and cured my problem.
However I’d still say the points above are most likely problem.