Well, just a quick update;
I have started to pull down my motor. Everything that has come off is actually in really good nick! Does not look like the engine has been apart before. Intake manifold / Intercooler is clean. No gunky residue inside. VF28 turbo , no shaft play or damage to fins.
Still a bit confused as main issue is 0.00% compression on number 4.
Turned crank manually - all valves moving up and down.
Pulled head off number 4 - piston looks fine from top view. Block slightly scored but nothing out of ordinary.
Top of valves all look in place and seat well, little carbon build up but nothing to be alarmed about.
No sign of head gaskets leaking.
Have noticed one bearing is completely screwed, quite a lot of play when you give it a shake.
At this stage I thought I would have found a chip in a valve or a gaping hole in a piston or something. Nothing, almost concerning I have a major problem, taken motor to bits and have not found a cause yet- not much more to take apart!
Am just about to take the head off the the other side and fully disassemble the block. Will update soon.