Spring plates are the ones off the original shocks.
As an experiment, I used a spare spring plate and some rubber to space it to replace the top cap, meaning it clamps down on the tower tight, rather than having the gap. This stopped the knocking!
I know its a bodge, and I have no intention of leaving it like that, but that says to me that there must be something wrong at the top!
Oh and its definitely no issue with the top mounts, I've replaced them and the bearings, but it didn't change anything.