Diesel hard cold start / lumpy idle / over fuelling?

[tpnofc]

Car is a 57 reg 118d and for the last few months it has been struggling to start on cold mornings, or when it has been standing for longer than 4 or 5 hours. There are two fault codes logged, and they both come back as soon as they are cleared - 4a74 and 4a79 which both relate to "glow system - communication". Considering the plugs and the controller have been replaced and these codes are still there, I assume there must be a shorted wire or electrical problem somewhere - has anyone else had any issues like this before and may be able to help work out what could be wrong?