I tried a bit of google, but almost all of the posts were either that the bike had been dropped previously or under-inflated tyres. I did see something about uneven wear on the brakes (There are 2 on the front of the STR).
I had a bike mechanic mate look over it and then kicked myself!
My tyres are Dual Composite - Hard rubber in the centre, softer (Stickier) rubber on the edges. the softer bit wears out quicker than the centre, and the odd feeling was running over a ridge formed by the tread that slightly overhung from the hard to soft.
Mainly posting so it can help others