Club 80-90 Forums

I am coming late to the party here, but based on past experiences with other vehicles...

Pressurisation of the header tank,
Bubbles in the header tank,
Leaking radiator,

It all sounds like head gasket or cylinder head failure.
There does appear to be combustion gasses getting into the cooling system. - hence the need for a sniff test. Not rocket science or time-consuming, so I cannot understand why garages would not do it as one of their first tests.

Just keep an eye out - the higher than normal pressure within the cooling system would cause any weaknesses in the radiator and perhaps heater matrix to give up prematurely.
Same with any coolant hoses etc.

I think a thorough check-over of the electrics is also needed - not complicated, but quite old and possibly subject to age and corrosion related damage.
And possibly a previous owner faffing without really knowing what they are doing.

As for the coolant pump.
I would expect the temperature to rise quicker than usual when running, but other than increased steam and water pressure, I would not expect that much bubbling in the header tank.

i was having  kinda same issiue ,,, i tried bleeding the system few times issue kept coming back ,, i was away on a trip and took out thermostat and and binned it i carried on travling for abut a week hundreds of km with no overheating or cooliant light ,,when home i replaced the stat with a lower temp stat ,, while poking around i discoverd a tiny leak from the temp sensor for the glow plugs the O ring was rock hard and crumbled in my hand so i changed both sensors ,,no more issiues ,, i left the lower temp stat because i wasnt arsed change it again 18 months later no issiues to date  ,,,

E D I T 
also i discoverd the water pump was runnung backwards
after i paid a lad to swap the belts and pullys from a v belt too a rib belt set up ,, now thats ok depending on the pump ,, i've a 1y so this may not apply to your engine