I pulled everything down, checked all the fittings in the image train, I couldn't find any significant tilt with feeler gauges/micrometers.
Then checked scope collimation with the Glatter laser.
The dot missed the primary centre by about 20mm, and the return dot on the base of the laser barely hit the base of the laser!
I suspect the collimation was off by so much that it confused ccdinspector, which reported it as tilt??
I know I didn't re-collimate after re-mounting the OTA last weekend....
Such a rookie mistake!