OK, here's what I did. I got a modellers flat file which is the only file I have and which I've owned since I was a kid. I clamped it to the bench with the handle hanging off the end so it was totally level. I then put my iPod on shuffle and sat there for an hour working the servo mounts up and down the file doing my best to shave off material from the mounting face whilst keeping it all square. In the end I took off about 1.5mm so that the face is now flush with the lugs that sit in the chassis and brace. Job done.
