Sorry to gently resurrect this after a few weeks!
I finally got the chance to take the adaptor off and rubbed it down using a coarse grinding stone.
It made the rubbing worse! Despite using a micrometer to check I was taking both sides down evenly, the caliper was twisted in towards the wheel and the upper edge of the rotor was fouling on the caliper. (We were assuming that the inside edges of the bolt holes were parallel to the outside edges for the micrometer to be accurate) I ended up taking about 30 thousandths off, which should have been enough to move the caliper away from the rotor, but in the end I had to pop two washers totalling 40 thousandths in between the axle and the frame to get the wheel to turn again.
So, as I don't want to risk losing those washers during a trailside wheel change - I need another adaptor, with a smaller offset between the vertical, 'post mount' bolt holes, and the horizontal, 'International Standard' bolt hole faces.
Without resorting to trial and error, any ideas on a brand which is nice and 'slim'? This one is a Hope.