Digistat Rf3?

Any suggestions on an Rf3 Digistat, where everything is fine except for the room temperature display, which is stuck on 21. This remains the same when 1. we put an electric heater on right next to it or 2. we open all our external doors and windows instead. (Highly technical testing systems!) We have done the usual battery relacements and resets to no avail. As a result of this we are having to work the boiler manually. Are we missing a simple setting here or has the blooming thing shot its bolt?