What could be the problem?

The system is filled by water mains at the fill loop if your plumber has checked this its the only place for water to enter the system. However if you have a high presure cylinder fitted for your hot water this could be leaking into the heating line. Also check your expansion vessel as you know when you heat water it expands the vessel takes that expansion if it is full of water the system presure will rise but this presure seems to high for this tell your plumber to fit a double non return value fill loop hope this helps

N Ireland plumber
April 2009