R reg Rover 214si 1.4 not starting?

I have a R reg Rover 214si 1.4 which has low mileage, but the problem I have is that most ot the time it starts, but every now and then it wont. I use the key fob to unlock it, which also switches off the immob but it still doesn't start sometimes. The battery is fine, a new one has been put on it incase. It started fine, used it to go to work, then wouldn't start half hour later. left it and went back to pick it up 12 hours later and it started first time, drove it home, swithched it off, and tried it 5 mins later and it wouldn't start again. We're thinking it's something to do with the immob, but any suggestion's would be much apprecited.