I had the same problem last year... I ended up using 'flat-mate' puncture repair to get the car to a national tyres who managed to get 3 of the nuts off with a backing off socket. It sits on the outside of the nut and has a left-handed tapered thread in it. As you tighten it the thread bites into the nut loosens it off. It writes off the nut but works. You can pick them up at places like halfords for about £10 a set of three.
Took it to another local garage to sort the last nut n he who wanted to gas cut the nut off n write off the alloy, so i ended up drilling the head off with a 17mm tungsten drill. Takes hours to do but saved the alloy!
Good luck!