Glow-worm 30cxl?

Started off blowing 3amp fuse in boiler, water cold but heating working ok, engineer came out and fitted a service kit to the motorised valve, then the rads. kept getting hot even when heating was turned off at the clock, engineer came out and fitted a new motorised valve but the rads. are still getting hot when the heating is switched off, now we are being told it must be the plate heat exchanger that is blocked, could you tell me if this could be correct.