ideal isar he24 hot water just warm?

I have an Ideal Isar HE24 which was in the house when we moved in, the Hot Water is not getting as hot as it used to, sometimes it is hotter than others. Yesterday I came home and the main trip switch had droped out, I traced it to the boiler and by putting things on one at a time (the hot water followed by one heating zone then the other) it seems to be working ok for now but am worried it will fail again. Last winter I had a similar problem with the switch triped, I could not get an engineer out for at least a week so I checked a few things myself, found the problem was a stuck diverter valve. Managed to release it and everything has worked well since until yesterday. Question do you think I have the same problem with valve partly open to give hot water problem?