There's a company here in the US that makes mounting systems for these kinds of things, for iPods, phones, etc. (They call it a GPS here, not SatNav, But you knew that, right?).
http://www.ram-mount.com/
You buy a sort of clamp that goes on your handlebars. It has a ball/socket joint, and you buy a holder for whatever model of satnav you have. You still have to install wiring for power, but mounting the unit is easy. I have two bikes and I have the clamp on both of them, so I can put my GPS on whichever bike I'm riding that day.
