Aha..!!
Glad it's not just me then.
Changed my rears to a pair of Kumho KU39's. Under 80 it's fine, as soon as I'm into three figures and doing anything apart from going dead straight, the car gets floaty and gives a wiggle not unlike a very, very, very mild "tankslapper" if you see what I mean?
I've now done 500 miles and it's much, much better than it was. Not completely gone yet, but not far off. My tyre pressures are bang on, so as soon as I'm happy that they've bedded in 150% I'll start playing with pressures to find the optimum.
The wiggle was quite unnerving at high speed though, so if anyone's fitting new rears, drive with caution for the first few hundred miles.
:!: