I have managed to check for coil bind and I think I've ruled it out.
I measured the springs fitted into the head. Installed height with seat and retainer is 38mm.
I then put the spring assembly (again with seat and retainer) into a vice and compressed the spring to 27mm (cams are 10.8mm lift). Although it looked tight I could compress the spring another 2-3mm after this point.
Installed height (38mm).
Simulated full cam lift 10.8mm
So I'm assuming that rules out coil bind being the issue.