Solved - mechanic took it for a drive this morning and said he thought a wheel/caliper was sticking as it wasn't coasting very well. Got it up on the ramp and the front-right wheel was indeed pretty tight.
Popped the wheel off and the inside pad was pretty much down to the metal, he thought perhaps the piston was pushing too far because of this and not returning. Inside of the disc is pretty shagged too so it's having new discs & pads, caliper seems to be ok thankfully. He said the build up of heat would cause the disc to warp which was causing the steering wheel shake, and explained why it only happened at certain times above certain speeds.