Can't adjust headset correctly
Mystery solved today. I took it back to the shop and a second mechanic had a look. He suspected that the crown race wasn't seated quite right - it turns out the first mechanic hadn't faced the bottom of the steerer tube and there was a bit of paint there. The crown race looked quite beaten up so he had probably forced it on. The new mech prepared the steerer, re-seated the crown race, and its looking great now. Thanks for all the input.