I finally picked up the car from Williams today - I had to wait for a new windscreen and roll bar to be fitted (also under warranty) at the same time... I gave it a good few tests on the way home, and I can confirm that the hesitation isn't there anymore - it accelerates smoothly all the way to the shift light now Apparently the TPS settings were all out too, so they've been reset to the factory settings. That's made a big improvement to driveability, so I can only assume the TPS settings were wrong when the car left the factory.