I think (famous last words)... i've got it sorted. Only had a brief ride so the commute tomorrow morning should be more conclusive.
I basically took the front wheel off, retorqued fork clamps, installed front wheel and torqued up axle and clamp bolts. Not sure what was loose, but one of the steps seems to have improved things.
I guess gratifyingly sometimes it is the simplest thing, despite worrying about all the more annoying and complicated stuff.
Thanks all for the input. Appreciated as ever.
You said in your first post that the tyres are nearly new...... Possibly the the tyre fitter did things up in the wrong order or trapped stresses in the fork legs when he did everything up..