If it's just for the Bluetooth, you'd be as well paying £12.00 in
for one that just plugs into the cigarette lighter socket. I bought one of those for SWMBO's Fabia recently and it works quite well. It comes complete with a mic which you can either attach to your clothes or place somewhere on the car.
I also recently bought a Garmin Nuvi 770. Without an external mic., the sound quality wasn't quite up to the other Bluetooth kit. However, when it was 2 days old, the whole system crashed. I managed to get unlock codes and recover the maps but the Bluetooth has never worked since, in spite of trying almost everything that Garmin Support has suggested. I still have to try the hard reset, though....
As ever...YMMV