Why is my upstairs heating always on?

My heating system has a timed controller downstairs and a Drayton Digistat 3 Controller upstairs. Both appear to be working correctly but for some reason the upstairs radiators are constantly on even when the controllers show everything to be switched off. Normally, when the upstairs system is heating a flame symbol appears on the Digistat controller but now, even when this symbol is off, the radiators are still on. Has anybody got any ideas what might be wrong?