Why does my Zafira start but other times does not in particular in the heat?

My car will generally start ok but I've had it in the local garage and no faults was the answer i got! I had the RAC out on sunday as the problem continued and today the car just would not start when i tried to leave work. It has plenty petrol no problems with battery and checked with a battery pack. RAC said it may well be a problem with the immobiliser anyone had this problem due to the immobiliser being in the key & i was only given 1 key by the dealer when i bought her just over a year ago.