Oil pressure dropping when hot.

I have a 2001 Defender TD5. Despite less than 60,000 miles it’s been off roaded extensively and had much work done on it before I bought it including a new head gasket. People often said it didn’t sound right and then one day it cracked a rocker and bent a valve. This was fixed and the garage found some oil galleries blocked with sealant from the previous head gasket change. After the head rebuild it went well and sounded much smoother. After only a few journeys I gave it a high speed run for 1 junction on the motorway which seemed to be normal. As I came off and the revs dropped the oil pressure light came on. To cut a long story short I have found that when cold the oil pressure is OK, but as soon as it gets hot the pressure drops on idle so the light comes on. I have replaced the pressure switch and now fitted a mechanical oil pressure gauge via a T piece. The oil pressure is dropping to zero on hot idle. There is the right amount of the correct oil and there is no diesel in it. I have changed the oil filter and have checked the "infamous" bolt which was tight. Also there was next to no debris in the sump when I had that off to check the bolt.
It seems like its happening as the oil thins when hot. Also this did not happen before the cylinder head work and the pump can’t wear that quickly. I have seen reference to a valve of some sort in the head but can find no details in the workshop or Haynes manuals. I have read about an over pressure valve in the oil filter assembly but reading the workshop manual shows this to control oil flow to the oil cooler as the engine gets hotter, but does this play a part in restricting the pressure to 4 bar? If not that just leaves the over pressure valve in the oil pump or the pump its self. Can anyone give more details of the workings and specification of this over pressure valve?
Can anyone suggest other possible causes and diagnostics?

Take into account that the oil pressure switch is in the oil cooler housing then analize this

Stiffener Plate
The stiffener plate assembly provides lower engine block rigidity and utilises dowels to align it to the bottom of the cylinder block. A rotary oil pump is integral with the stiffener plate and an oil pick-up and strainer assembly is fitted to the underside of the stiffener plate. The stiffener and oil pump assembly is secured to the cylinder block by 22 bolts.

Oil pump

The eccentric rotor oil pump is integrated with the stiffener plate and contains no serviceable parts except for the pressure relief valve spring. The oil pump drive sprocket is attached to the front of the stiffener plate and is driven through a chain and sprocket system.
A pressure relief valve is included at the outlet side of the oil pump to restrict oil pressure at high engine speeds by recirculating oil through the relief valve back around the pump to the inlet. The relief valve and spring is a plunger type. When oil pressure is great enough to lift the plunger, oil is allowed to escape past the plunger to relieve pressure and prevent further rise. Oil is delivered to the pump from the pick-up through a channel in the stiffener plate. The outlet side of the oil pump delivers pressurised oil flow to the engine block main oil delivery gallery through a port in the stiffener plate.

I have no experience in engines i hope u can figure out something from here

Sometimes the pressure relief valve spring can break or become weak due to overheating allowing pumped oil to circulate around the pump and sump and reducing oil pressure to the engine, and the oil cooler where the oil pressure switch is, to virtually nothing. However if you have decent pressure when cold this does not fit the symptoms of a weak or busted relief spring which would cause problems hot or cold. The only thing I can think of that fits your symptoms is a cracked or loose oil pickup pipe assuming the pump is good.

I'd be tempted to take the sump off and look at big ends, pickup etc excessive wear in a bearing component can cause low oil prressure when hot- also worth looking at cam bearings

blocked oil filter (s)?
For the price of them I would change both the Rotary and Cartridge filters. They may be blocked and causing a restriction in flow?
A bit of a long shot, but you never know.

I have a nasty feeling that the engine has suffered some serious damage from the oil gallery blockage from when the head gasket was replaced a couple of years before I owned it or from the sudden drop in oil pressure. During the head rebuild done by the garage the cam shaft was worn and replaced with a used one. The hydraulic lifters where also knackered, and where replaced. I would hope any head issues where fixed at the same time. Prior to posting I took the sump off and checked the pump bolt which was fine. I also looked at the pickup which looked OK and the mesh was clear. I did not check the pump or relief valve at that time. The oil which had only done a hundred or so miles since being changed when the head was fixed seemed very black for so little use. I changed the full flow filter but not the rotary one. Both had been changed with the head work. When cleaning the sump out I found a few aluminium chips. They were too small to identify but I was hoping they were bits of the broken rocker which was replaced but I will never know.
As it happens the rear diff has a problem and I have just removed it to get it looked at by Crown diffs so I can really test it properly until I get that back. I have run it and the pressure is fine but until I can work it to get it properly hot I won’t know what is happening. Once I have done this I was considering getting some oil analysed to see if anything shows up like big ends. Has anyone got any recommendations of who can do this for me? The other thing I was considering was changing the oil filter assembly which apparently contains an oil pressure regulator. Is it possible to see much just by removing this?

id say big ends worn oil gets hot gets thinner and with its history id expect wear