I have an unvented Megaflow system and I have noticed recently that the water does not stay hot for a long time after the boiler (3 year old) has gone off. For example in the evenings it' on between 7:00pm and 9:00pm and without much water being used it's cold by midnight. Anybody know why this is happening? I have the Megaflow system for 2 years and I have never got it service, now planning to get it service within the next week or so, what do I expect from a service and what would be the average price of a service? Thank in advance.