Wny are the radiators always on?!?

Hi
Whenever I switch the hot water on, even with the heating switched off, all the radiators come on too. I control it by turning off the radiators individually, but this doesn't seem like a good long-term solution. From reading some other questions it looks like it might be a 3-way valve? Is this correct? If so, who should I contact to fix it - a plumber/electrician/corgi?
Thanks for your time.

Hannah
April 2007