I'm going to try taking it back tomorrow.
Pr, what alternative mount did you use? It was the Tom Tom alternative mount kit which caused this problem. There's nowhere flat enough on the dash for the windscreen-alternative disc to stick, so it soon fell off there, so I used it on the windscreen. But the length of it seemed to mean that it vibrated more, and therefore dropped off.
Maybe I should have lick-n-sticked after all.

Silly of me to not want gob all over my windscreen.
Anyway, now I have the mount from the kit, I can start looking into other things I can attach it to, like the Kuma.