Thanks for all your replies. I tried to use a sucker mount on the radio fascia with my HTC phone but it kept falling off. I think the HTC screen is too small anyway and as has been said it is a bit dangerous looking down at the satnav too. I think the windscreen is too far away for a satnav hence I was looking at vent mounts as they are quite high up and that led me on to the TomTom website.
I like the look of the Brodit holders - I'll do some more research.
Thanks!