Weird GPS issue

Hi all, 

I've had my A6 Avant since July, it's a 15 plate 3.0tdi and I'm loving it so far, but I've got a weird issue with the sat nav.

I don't use it very often, but about 75% of the time it doesn't work (picks up 0 satellites) and when it's not working, it also seems to interfere with GPS on other devices like phones etc. I don't really need the in built system as I use Google maps anyway, but is it likely that the aeriel is faulty and causing interference, and if so will unplugging it help? Or is it more likely that a previous owner has installed a GPS blocker?

I used an app on my phone to check gps signal, and standing next to the car I have a strong signal, but as soon as I press the unlock button the GPS signal disappears. If I sit in the car with the ignition off long enough for the electrics to cut out again, the GPS signal on my phone comes back

Thanks in advance