Worcester bosch 2.8i junior, cuts out with red flashing light?

My boiler has been working ok through the summer, heating hot water on demand, but now its getting time to put the heating on there is a problem. The heating will work fine if no one turns on the hot taps, if they do the boiler cuts out and the red indicator light flashes, if the boiler is reset by holding the button down for several seconds, it all works again until a hot tap is run again.
Any ideas?