The windscreen ones are ok, I had the same AFAR one as Martin, reception was ok at best, but replaced it with a short whip roof mount aerial from a new polo with a nice long extension cable (about £15 off ebay) to the headunit. Reception is excellent with little drop off. It is a bit more expensive (about 3 or 4 times) but nothing in the window and far better signal an therefore sound
P.S - once you get the RCD get yourself a Bluetooth kit or it (5k0). Proper piece of plug and play kit