Satnav not working?

Audi A6 avant 2009 and the sat nav indicates we are somewhere in the English channel. Audi want £600 to fix the problem- no way will I pay that for a not very good satnav. My wife's Hyundai i35 has a far superior satnav. Tried the suggestion of getting into engineering mode and it did not work.
Will try getting a satnav aerial off e bay and see if that works- any other others