Pretty sure it's not a head gasket, there was no smoke previous to this.
I checked the earth, but I can do that again - clean and re-attach.
When you refer to undoing the "inlet piper from manifold" - which is this?
The inlet was fouled up with deposits, it wasn't pretty. I took the EGR off, and throttle body, and cleaned and refitted. At that point, the car ran again (but rough, with no power, as if it was stuck in limp mode).
On my 159 1.9jtdm the air intake to the metal inlet manifold is a rigid plastic pipe of the end of the manifold held in with 2 Allan bolts.
Undo it. This elliminates any blockage on inlet pipes.
You can also shine a torch down it to check for carbon blockage.
Are the manifold swirl valves an bar ok.
Turning the engine on the crank pulley will tell you if it tells ok or mechanicly not ok.
You could still have a sensor issue.
But get the basic mechanics out the way.
You may have toasted your starter but that would be a symptom not the cause based on previous posts.you could also spray some easy start in the manifold whilst pipe cracked open.
Would be Interesting to see if it runs on that for a few seconds.
That would point to a fuel issue. If your getting fuel pressure then fuel timing is off which would be based on crank/cam sensors