Hi,
after some thinking i have put a large O-ring (from my toolbox) between the holder and the bracket and then had a short session across Bodmin moor yesterday with my lovely phone on the holder. And guess what? Didn't budge! Got well shaken though.
Also took the opportunity today to test how waterproof a simple plastic bag is. 100% i have to say. After 40 miles through rain and sunshine today it stayed dry inside, apart from some condensation which blurred the picture a bit.

Rob