Home alarm installers ?

Can anyone recommend a good reliable home alarm system installer in the Marlow/Beaconsfield area ?
The issue is an existing installed system has not been used for a few years and it needs a complete overhaul and service - probably a new keypad too. We are looking for someone to look at this and advise. Someone has already been along - charged to a call out fee to come and look and now recommends a whole new system. We aren't buying into that.... so need a bit of tosser advise on this. Thanks

Alarming

Probably needs new batteries in the main box and the bell box key pads last for ever.
Dave GS