Potterton Ultra 2 - re-pressurised, but pressure drops within 2days?

We've had the boiler serviced, clean bill of health. Then had a leak from the water cylinder, so that system replaced. Engineer re-pressurised the boiler, and the central heating worked. Within a day the pressure had dropped again. It comes on for about half an hour now, then cuts out again.

Who do I call to fix it - ie, what do I tell them thats wrong. Is it a leak somewhere?