Potterton Suprima/Suprima L. constant central heating?

We cannot get our central heating to switch off unless we switch off the boiler. When calling for hot water only, all radiators in the house come on, we have tried turning the thermostat off in the hall but still full heat delivered. This has happened twice before but i managed to rectify fault with reset button but this time to no avail. House is 5 years old but as we've only resided here one don't know if this is a replacement or original-help it's roasting in here!