Thanks you very much for the reply, comments bellow..

Check the battery with a voltmeter:
Done, all good. It turns over fine, so I am sure it is not a battery issue.

Try using Shell superdiesel for a while, or add a diesel inject cleaner
Done, put £40 of shell diesel in the other day as well as a bottle of injector cleaner in the last 2 fill ups.

Try turning the car on but don't start, turn off after about 15 secs
Done, makes no difference (good idea though). I had it booked into my local garage and he thought it might be glow plugs as well. Unfortunately, it is not.

Check the battery connections
Will do.

I took the MAP sensor out the other day as I had read that this can become full of carbon and cause issues. It was indeed full of carbon, but giving it a good clean has done nothing.

I have also noticed recently that under breaking the engine has a noticeable shudder to it. Can't help but think the 2 are related.

One last thing, I have a OBD2 reader. Are there any parameters I can look at that might give me a clue as to what is going on?

Edited by MechanicalMan on 23/12/2021 at 21:57

There are 3 parameters to be looked at with a cold engine. Firstly, engine coolant temperature should be similar to outside ambient temperature. Secondly, engine cranking speed needs to be around 200-250 rpm. Next is fuel pressure, needs to be a minimum of 180-200 bar. These are basic tests. If these are within spec, then further tests will be needed. It's possible the injectors will need checking.

Thanks for the update, it might help someone else if they hit upon this thread when searching. Let's hope the mouse enjoyed its drink for all the aggro it caused...