OK thanks - I've changed the setup around now and it seems better. The disc spins fine and the pins and free to move in and out. The pad is worn so I have a new one arriving in a couple of days. One question I have is whether the brake should work in neutral? Or does it only work when the mower is in gear? I ask because when the mower is in neutral the disc doesn't spin at all, but when in gear and I turn the wheels it does.
Thanks