Kierank

Hello everyone, I have just bought a spares or repairs 12 plate Legacy NavPlus 2.0 Diesel and would love to get it going again rather than break it down. The seller said the serpentine belt snapped on the motorway, it was recovered and the belt replaced but now the car will only turn over with all the electrics working but will not start, there is also a dpf light flashing. Engine is turning via the chain. Where should I start to find out why it isn't starting up?