Sounds like dodgy GPS reception. Do you have a satellite symbol (small icon of things going round a planet - looks a bit like the pictures of atoms you used to get at school) in the top left of the display?
If you can see the icon it means that there is no signal, eg as you go through a tunnel the icon will appear but then disappear when you come out the other side.
Yours sounds like it isn't receiving GPS at all (or intermittantly) and relying only on gyro/wheel rotation data...
The symbol means that the device can't get a fix. Mine gets that symbol when it's in the garage (brick walls & tile roof blocking the signal). Out and about, it goes away. It may be as simple as the connection to the aerial, or you may need to reposition it. My 2006 saloon has no "shark fin" on the roof, so it must work with the aerial inside the car, but maybe you'll need to move yours around a bit.
Right, I`ve been and moved the antenna to outside the boot window hatch (it`s an estate) as a temporary test measure and the nav works perfectly. So presumably it`s merely the positioning of the antenna that is at fault.
Does anyone know where the antenna is mounted on the estate please? pics if possible!