Thanks for the reply. I've removed the head and found, just as in the previous four times, the water passages in the head have corroded and breached the gasket. Once I've had the head repaired, (hopefully), I'm changing the type of antifreeze that I've been using as clearly it's not preventing the alloy head from being eaten! I've already flushed the rad and block thoroughly, so will try the red stuff next time around.
One question I do have, does the engine really need to have water flowing through the inlet manifold? I live in a hot climate, so cold weather is not a concern. I've been thinking of blanking off the water inlet to the manifold while the head is in for repair thus doing away with a lot of external pipework. The heater has been disconnected for years, by the way.
Thanks again for taking time to reply.
Ooh! The heater being disconnected could be significant.
As a general rule, when the engine is cold, and the thermostat is closed, the only way water circulates is through the heater and back to the engine. As it warms, the thermostat opens and allows some of the hot water to pass to the radiator to be cooled. There is no water valve on the heater, it will be a full flow system, relying on the flaps inside to determine whether you get heat or not. Earlier designs, before the mid/late seventies used water valves, but needed a separate part of the circuit that allowed the water to circulate within the engine until the thermostat opened.
With the heater disconnected, if you have joined the two hoses together, then water will still circulate, but with a shorter distance, so should not be a problem.
But if you have blocked off the heater inlet and outlet from the engine, the coolant will not circulate until the thermostat opens. This can create two problems. First the coolant is not moving, so the hottest part of the system, at the head, will get significantly hotter than designed before the stat opens, as it will be relying on heat transfer rather than the hot water movement. (I wonder if the hotter spots might accelerate the deterioration of the coolant.) The head will run hotter which can lead to earlier failure of the gasket. (Gasket failure might be the cause of the head deterioration, not the result. The second problem is that with the water pump running, but little circulation, you may get 'cavitation', where the coolant aroound the pump impeller bubbles up, like being whisked. Free oxygen in the water escapes, creating bubbles and potential airlocks and hot spots.
Lots of theory, difficult to prove if any or all of that might be happening, even if at the car, but you should consider reinstating the heater circuit to standard. If the internal flaps in the heater box are good, this should not cause any heat issues in the car.