If the discs were warping, there would have been vibration when braking but not under acceleration. How much were they warped (how much runout when checked with a dial gauge)?

Driveshafts or hubs seem a more likely cause of your vibration problem.

This sounds interesting. I find it hard to believe modern discs warp with normal use. Our second-hand 2000 Focus had an unnecessary new set of discs and pads at 29,000m when I bought it and the discs are still serviceable (new pads at 83,000) at 106,000. This is an interesting link...

www.mossmotors.com/SiteGraphics/Pages/brake_discs....l

Could it be a slightly eccentric drive shaft bearing? Perhaps there is a batch of vibrating SEATs out there?

Hello - I have more information that might lead to a diagnosis. I had swapped back to front wheels in case there was something with the actual wheels. But same basic problem. I then had them re balanced in order to 100% ensure that the wheels were not the problem at all. Immediatley the tremor disappeared from 2nd and 3rd gears (at speeds you'd use those gears for). But then it came in aggressively at around 60 mph and disappeared at 80 mph (in 4, 5, 6th gears). Very much so under acceleration but all but disappeared with no acceleration/foot off. Some wobble under braking at 65-75. And it varies - not to do with road surface though. At one point I thought it had stopped doing all this. But then was there but slightly less so. All this was on a trip to the Netherlands over the last 4 days. As you know, the roads over there are very smooth so a perfect testing ground for anything to do with this problem. Any thoughts - I'm faced with disposing of this much loved car at a knockdown price!