Does the boiler go to full flame with hot water only selected when you turn on a tap?. If you only get hot water when the CH is on the DHW microswitch may be broken (quite common).
Or...With hot water only selected and the hot tap running (and burner on, assuming the DHW microswitch works) do the radiators heat up?. If so suspect the three port valve is due its regular service. The parts come with the excel/eclipse service kit.
c
March 2008