Hello, I hope the following assists you. From the description you've given and if the engine continues to run but won't rev at all then there is a fault with the instrument cluster. The instrument cluster is part of the immobiliser system on this model but it is also a gateway for the different speed networks on the car, the signals from the accelerator pedal go through this to the engine management. Sometimes when driving with this fault the immobiliser LED will start to flash and the engine will just idle. A new cluster is very expensive from Ford and needs to be programmed to the car but there are companies that can repair clusters and the immobiliser doesn't need programming after this, an example is www.clusterrepairsuk.co.uk/repair/model/ford-focus-instrument-cluster-speedo-mk2 A DPF fault wouldn't stop the car from starting unless the exhaust temperature sensor was open circuit but this would mean the engine wouldn't start any time. I hope this helps but if I can assist further let me know and I'll reply as soon as I'm able to. Best wishes, Sean.