System lost its umph.

We have a Range Powermax 185 with pump/prog/hydro, open vented.  A couple of months ago, we noticed the radiators were running cooler, and the system had difficulty heating the house. The hot water is cooler as well, and you can't get a hot bath.  The burner still fires at intervals, but seems to keep the store at a lower temperature. I've now found that when it thinks it's up to temperature and the burner is off, if I move the rocker switch up to hot water, the boiler immediately fires up for a nice long burn and showers and baths are fine again.  Switch the rocker down to timed again, and the burner stops, and both heating and hot water become sub-standard as heat is taken from the store by heating or drawing hot water. It's as though there are two thermostats, though with one heat store covering both CH and HW I wouldn't think it likely.
Any ideas would be much appreciated.