Pump running all of the time

I have a 20 year old simple pressurised water heating system comprising 3 south facing panels, and a large water storage tank. It's controlled by a RESOL DeltaPro unit which has worked very well, switching the pump on when the roof temperature exceeds the stored water temperature by the set amount. At some point recently (it's winter) the pump has started running constantly and I'm not sure why. There's no error report and none of the operating parameters have changed. As well, though I suspect not related, the system gradually loses pressure which is likely to be a plumbing problem rather than a control one.

My thoughts are that it may be a faulty relay in the control unit and that a replacement will solve the problem, but before I jump in I thought I'd seek advice here.

Thanks

Mike