potterton performer 28 leaks water?

when heating is switched on, the boiler seems to leak cold water from around the open 15mm pipe, under the assembly and behind the lower control panel.
seems ok when just running hot water only, if this has anything to do with pressure settings then I would be gratful for any info on how to fix this fault.
this is a potterton performer 28, fitted about five years ago.
many thanks.