Handlebar twitch when braking - any ideas?

Damn, that sounds really plausible. I should have considered it more
carefully when John Heath suggested it up there ^ somewhere. I'll have a
close look at both forks asap. Thanks

 

There is obviously something loose - the disc being on the left means
that when you apply the brake it's pulling the left fork leg forwards
compared to the right one, causing the twitch to the right.

I'd go for the left fork leg flexing in some way under the force applied
to it by the caliper gripping the rotating disk - maybe the new fork
bearings aren't in right?