They often have a motorised valve that sends the heated water from the boiler to a heat exchanger coil in the hot water tank (to give you hot water) or to the radiators to give you warm rooms - yours may be stuck in the wrong position.
http://www.centralheating-diy.co.uk/images/3_port.jpg
You can sometimes operate them by hand with a little lever.
A heating engineer will soon sort you out - I would guess a bill of about £200 but that's just a guess - could be a bit more.
In the mean time use the immersion heater if you have one.
Good luck...
Peccavi
November 2009
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