Martyn, while I have no idea what a cap/bat is there might be a good reason why it has such a short range.
The frequency spectrum is rather limited at the frequency range you are talking about. Increasing the RF power of a device would also increase the interference it could cause to other equipment.
The makers of this equipment know this and limit the RF power! Rf interference can be a big problem in our electronics dependant society.
So, for an easy solution you could try to improve the antenna's to get a bigger range without a RF power increase.