Potterton Performa 28 hot water / central heating issue?

I wonder if any of you can help please? For some months now, whenever I run hot water, the central heating comes on as well. The hot water goes through a cycle of luke warm, hot, tepid, luke warm, hot, tepid and so on. Last year as there was an issue with the internal expansion unit, valve or whatever it is, the engineers fitted an external expansion unit the size of a beach ball, to maintain the pressure. I've noticed over time though that the pressure in the boiler still drops lower than 1 occasionally so I increase it to just about 1.2. I've also noticed that when the boiler is at a lower pressure, the water comes out only tepid and never gets hot, so I overcome that by increasing the pressure to increase the water temp.

I'm not an engineer or plumber, just a user but having been present at most of the annual services and fault repairs while the boiler was covered under a policy. I didn't take out a policy this year so need to buy the required part and get a qualified engineer to fit it, so rather than go with my gut instinct that it's the diverter valve, I need your expert opinions please!

Thanks to anyone in advance for assisting.

Sara Bains
April 2011