Starting problems

I own a 51 plate hatchback magane 1 this is my 1st time on a forum so bare with me My problem is I can leave my car after driving it for up to an hr plus and when I return the car will not start 1st time other times it will I've had a diagnostic on it and no faults was found the garage told me it's the immobilser but dashboard illuminated is not showing a fault with immobiliser I've recently had it serviced so I know it's the usual problems spark plugs etc so can any one help plse thxs