I'm "lucky" that mine is still exactly the same spec as when it left the showroom, so excuses like that shouldn't arise. We'll have to see, as I promise to keep you informed.
Just back from a run tonight to see if I do get the vibration in fourth - yes I do, flooring it up to 85-90 in fourth (5k revs) and the vibration is definately also there, so I can confirm your observation, Mole. Changing to fifth doesn't seem to make it any worse, thought. It just seems to stay constant.
As far as I can tell, it's related to road speed and that's all. I had thought maybe something to do with gearbox, but now that I've observed it in fourth and fifth, I'm not so sure. Maybe if it was an S2000, I would be able to hit that speed in third to completely rule out the gearbox!!
Anyway my current thinking is that perhaps some paneling is loose underneath - it's that sort of vibration. I know that Polo is suposed to have all this aerodynamic stuff underneath (i.e. panels instead of wax for corrosion as well) - but I haven't looked under my car yet (should I admit that in this company!!??!!). There is a huge lump of plastic that continues from under the front bumper to in front of sump - perhaps this isn't very secure?
Does anyone remember the mk2 Mundano when it first came out - S, R, P reg - did anyone notice that the early ones - when they went past you at 90+ had the bottom of their rear bumper "seriously" oscilating in the airflow - you didn't see it in later ones, so ford must have made a design change. Sometimes, you'll see these Mundanos with the bottom of their rear bumper missing - fatigue/cracking of the plastic I guess due to this.
Anyway, I'm guessing if its a result of a fundamental design decision of this nature - then the chances of getting it "fixed" are unlikely.
One the plus side though - fantastic run tonight. Roads were deserted. Down the side of the mway through country roads, onto the mway for the high speed testing. Perfect. The speed you can carry through the twisties in this thing is astounding. It just keeps gripping. And the brakes seem to be pretty fantastic too, now that it's worn in and I'm starting to push it properly. I've just fallen back in love with the Polo, despite these obviously "bad vibrations". Next week though, I'll be back to hating it - just wait and see!!
Deek.