Bongo Fury

From what I can tell you only have a couple of options and I think they've all been covered above.

If you've got too much pressure, then you are either pressurising from exhaust/combustion gases entering the coolant. A sniffer test would prove this.

Or you have a faulty coolant reservoir cap. The caps have a valve in which lets air in as the coolant contracts/cools and releases air when it expands/heats up.

For less than a tenner if be trying the coolant cap first.

You could then try a DIY sniffer kit, to check for gases yourself.

Perhaps a compression check to make sure all cylinders are the same and you haven't got an obvious one that is missing pressure to the coolant system?

Thank you. I have now removed the cylinder head ; it is with a local engineering company getting pressure tested. I hope to find out the result on Wednesday

Photo of cylinder head attached. Apparently it is not cracked, which is good news. My research points me in the direction of a massive air lock, if you spot the similarities in the two attached photos.

Surfkernow: "But, as soon as I replaced the thermostat, and changed the coolant, it starts everytime without fail. I can't see how the two are connected, but it was odd."

Theres your problem, looks like the self bleed kit didn't work too well, was it a 'rusties'? If so you just saved me fifty quid.

As for that clicking starter I don't reckon theres any link with the welding or your coolant problem, I think your starter motor contacts are failing as that was exactly what my hilux did, then it would work again for weeks and then 'click' for no other reason than it hated me. I got a repair kit off an ebayer which were a set of contacts/brushes and replaced the old set once I'd removed knuckles getting the starter out of the truck and it never happened again while I had the truck.

Its worth doing while you've got stuff apart as its only going to re-occur and let you down when you're away from home in it.

Yeah the CH does look very similar to the smaller pic of the other one.