I have just bought a drayton digistat +3 2208 programmable room stat to replace the original TR2 which has 3 wires. red to F. blue to 3. and yellow to 4. the new stat has 3 terminals C, on, off. and the diagram shows i only need to connect to C and on any idea which wires to which terminal. My boiler is a greenstar R35 HE GC No. 47 311 80. Can anyone help? Ian