Do you mean Potterton?
If it is a combi-boiler, there are a few things you can do yourself � and save yourself a bundle.
You need to find where the water is escaping from your system. You can look just as well as a plumber, who is charging you �50+ per hour.
Locate the boiler pressure relief outlet, which should be a pipe venting outside, reasonably close to the boiler. Check that it is not dripping water, place a clear plastic bag over the outlet (held in place by a rubber band), such that it will catch any water from the pipe. After 24 hrs check whether any water has been collected.
While waiting for the plastic bag to catch water, closely inspect all areas within the house, close to the hot water system (radiators, pipes, the boiler etc). Check for any dampness on carpets, walls or damp patches on ceilings. Once you find the leak � you can begin to think about getting it fixed, having saved the time the plumber would have charged you.