Engine runs again
Let the oil pressure come up on the starter again without plugs. It was tad rough to start with as it tends to be.
Turns out the fan temp sensor switch is broken which is why the fan wouldn't run when it was hot.
So the problems were dodgy earth, broken fan switch and the starter stayed engaged.
Solutions: New earth strap, new fan switch on order, greased starter mechanism, new oil, new filter.
I may need a new starter, the alternator could do with being replaced as well.
All in all lucky escape I think.
Once I have new bits I'll be able to take it for a run make sure it OK over a longer period.
EDIT: Forgot to say I did a compression test as well all within a smidgen of 12bar over all 6 cylinders.