Thanks for all the responses,
Ok, for definate I know its not low fuel, low temp, service needed or a reversing sensor (not fitted).
I thought too that it may be something to do with the stereo. But I don't think so now because it would be too much of a coincidence as I just changed the headunit a couple of months ago, and for the two units to make the same noise so randomly, nah.
The alarm was another choice of mine. Its just the standard alarm and there is no tracker fitted to the car. Where do I find the battery that could need changed? But again, the beeps are so utterly random when they happen, I'm thinking the alarms battery is probably ok because the beeps would happen more often if it wasn't.
Someone above suggested the car could be haunted . . . .
I wonder what it could be?