Early 90s Toyota diesels were notorious for cooling problems. The 2.4 2LTE engine was infamous to the point of some garages refusing to touch them.
The best starting point is almost always to replace the radiator cap, as it may be letting pressure out of the system. At the same time, it's worth pulling the thermostat out and putting it in a bowl of boiling water to make sure it still works.
If neither of those work, and the water pump is OK, you can flush and refill the cooling system to get all the gunge out. Check also that the hoses to your coolant header tank are intact and not blocked.
If all of that fails, the cylinder head is probably cracked and you will be looking at getting rid of it. A new head will be over £1,500 fitted.
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