Del.. I.m assuming that the thermostat control knob is snapped off from the shaft, which is quite a common problem with the 24i and 28i boilers, the thermostat knob will spin all the way round without any resistance. Catch hold of the knob and pull it towards you and it will release, you will see the remains of the shaft in the slot this is D shaped. Sometimes it will push through and fall out inside or a small tweezers to pull it towards you, note the shape of the shaft on the new control knob (D) locate and push it will click into place. Knock off electricity to the boiler when poking with tweezers.
Darrengas (heating engineer), June 2009
