My P Reg Diesel Citroen Berlingo won't start?

My stepson has acquired a Cheap P Reg Berlingo. It drove home fine but, the floor had a load of water in it and the previous owner told him that the heater matrix was 'nadged'. The lad got a new matrix and mainly by himself, took the dash apart then got a mechanic 'friend of a friend' to fit the matrix. All back together the thing wouldn't start. He's now bought a new battery and...................it turns over but won't catch!! Sounds then like the starter motor runs out of puff and winds back off!! Any suggestions what the prob could be please?? I had wondered whether there could be an immobiliser???
Regards in advance
Malc