Low compression in one cylinder

Hi, unfortunately my td5 took in some water and perhaps other rubbish in a while back due to a failed air intake pipe (causing the foam filter to collapse to one side).

Turbo fins were damaged so have replaced that, but the truck still takes a while to start, has a rough idle and emits a lot of white smoke at low revs. It does seem normal with more revs, although slightly down on power. There is also excessive back pressure in the head which is noticeable when you take the oil cap off.

I’ve compression tested it, cylinders 2,3,4 are all 400psi where as no. 1 is at 200psi.
When cranking over it makes a different noise to the others, almost like air being pushed out?

My thoughts so far is it could either be the head gasket between cylinder 1 and the timing chain or it might be bent rod?

Has anyone any other ideas? If it is a bent rod is there a way to check without dismantling the whole engine?

Thanks

When I was working on my 200di I left a rag in the inlet manifold before connecting up the filter it had rough running , noisy with sound coming back out of filter, compression was less than 100 psi
the rag got stuck in the port stopping air getting in hence low compression
if it’s sucked in water and hydraulic locked , that’s lucky if it’s one cylinder , but amongst the usual suspects may be unusual ones.
might get a bigger response to this on the defender forum on these newer engines