So the light came on, changed the MAF sensor as they are the first port of call usually.
Still on and so changed the fuel filter as it hadn't been changed for a bit. Still on.
I bought a fault reader off ebay and that showed no faults.
Took it to local garage and they came up with 5 faults. Cleared all them. Went to drive away and the light was back on. Diesel pump control unit, so I got a ebay pump and the garage fitted just the control box off the top of the pump. All seems well and it has its MOT this tuesday.
So always worth getting a T4 or a proper reader on it.