Okay, great that helps to know what's going on.
The oil is clean, there's no milk shake in the filler cap.
Not had a coolant test for combustion gases yet.

 

Yesterday I removed the radiator and took it to my local radiator specialist for a flow tested.
About 80% blocked.
A new radiator ordered.

 

Glad you got confirmation on the issue!

 

It’s always reassuring when you eventually find that elusive issue causing you some strife

 

Continuing on this issue, new radiator fitted at 12:30 today. 10 litres of coolant is the maximum I can get into the system. While the radiator was out I flushed the block as best I could plus removing the heater hose from the waterpump and flushing this as well.
everything was back in by 1/4 passed the hour. Started the truck up, had it running until 5pm tonight. Temp gauge sitting at the half way mark. I can remove the radiator cap without and problem. Thermostat must be opening as heater working. Then I noticed the heater went cold. Upon revving it up the warmth coming through again and at idle, tapers off to luck warm.
Temp gauge still at half way. Coolant can still be heard running through the heater matrix. I had the truck up on axle stands at the front to try and allow coolant air to escape but there is still air in the system. There is no bleed screw on the pump or any other part of the coolant system.
The engine is clean, and there's no gasses getting in the waterways.
I guess, its just a matter of letting it self bleed over the next day or 2.
I haven't driven it yet.

 

Once the engine warms, the cooling system should be under pressure, and pulling the cap can be rather exciting/dangerous......

 

Yep, been there, got the badge
Certainly no pressure when lifting the red clip on the cap, even if I take the cap off. All the hoses are hot

 

Let it cool, put the cap back on, then let it warm up, with the cooling system sealed, it *should* build pressure, if it doesn't, then you have a leak somewhere.

 

Further running this morning.
Brought her up to temperature, checked coolant level, removed the cap, no pressure whatsoever. Then I saw the coolant level drop and empty the header tank.
it took another 4 litres to bring it up to the marked level.
Left it running for another 35 minutes then removed the cap again. Again no pressure at all.
level still at the correct mark.
Got back inside and revved her up to 3500-4000 rpm for a 60 seconds, the water gurgling noise seems to have reduced quite a bit.
Turned her off, left it for about a minute, removed the cap, no pressure, not even a hiss. Water level still at the correct level.
I'm keeping my fingers crossed 🤞

 

Further up date again.
I replaced the new radiator cap with the old one that was on the truck when I bought it. It now builds up pressure inside the system like normal and doesn't spray all the coolant out of the overflow anymore.
1hr 30 minute drive both outskirts and town center traffic with no issues apart from the heater warmth being intermittent.