I have an old Potterton Flamingo with 2 motorised valves, which is pumping over. Probably caused by a blockage somewhere. Im a bit confused as to why the overflow from the expansion tank drips more outside, when the hot water is off. Heating hasnt been on for six weeks. Hot water was turned off at midday but drip from overflow now seems to have got steadily worse through the afternoon, and is now at its worst at 1900 hours. Any ideas please