I have the Haynes manual for my fabia and was trying to find the wiring diagram for the heated seats which does not seem to be in. Does anyone know for a diagram as my drivers heated seat will sometimes work and sometimes not any advice please.

I think you'd be better to start with the Rheostat in the dash before checking the wiring. Have you looked at the connectors under the seat, IIRC there may be a relay under there too.

Better still a diagnostic check might give you more info.