Water supply constantly being supplied to boiler???

Hi,
We have the water supply pipe in our kitchen which is situated by our patio door, when we turn the taps on, you can hear the water coming through, all normal, except that it has all of a sudden started flowing constantly, as if a tap was turned on - except everything is off??

Standing next to the boiler, it sounds like the water is going through one of the pipes into the boiler, even though the boiler isn't in use, and even when it's switched off. There hasn't been any change in the system pressure

It started yesterday morning and is still going!

Any ideas what is causing this too happen, and more importantly, any solutions!!

My boiler is a "Ideal Isar HE30", it was installed in oct 2007, and last serviced oct 2008

Many thanks in advance!!

Kev