it is not unusual to call it an explosion - suck-squeeze-bang-blow - but because of the glow plug issue, IMO it seems important to think of a controlled burn even in a diesel, and cold walls and head prevent a proper burn (same can be true in a petrol engine in cold/damp conditions)
Woohoo, all faults gone (for the moment at least). They replaced all 4 glow plugs which came out looking very black. Thanks everyone for your help and advice.
