It seemed to appear after a sprint up the M40.... I've had a quick looked and couldn't see anything obvious and decided it must be subframe bushes or something similar... Anyhow, having not had time to do anything about it, I went off on a trip to Ireland and while cruising down the A48 towards fishguard everything got suddenly worse and I thought I was bound to miss the ferry!
Turns out I'd had a blow-out on the offside rear which more or less shreaded the tyre - closer inspection revealed a serious bulge on the inner wall and it must have been that which was causing all my problems. Since I've had the spare on there's no vibration at all.
The moral of the story - check the simple things first....
Incidentally, I replaced the tyre whilst in Ireland at this place that looked like a shed and I only had one done cause I thought it was bound to be expensive. I should have bought a whole set as it was 85 euro for a Goodyear Ventura - my local tyre shop wants the same in pounds!!