Thanks to all who contributed advice regarding the coolant ejection problem. I think that the problem is now solved so here are the steps that I took to isolate the problem, just in case there is anybody out there who may make use of it.
On my engine (late '92) there is not a bleed screw at the rear of the engine. There is a short rubber pipe at the rear end of the block to the thermostat housing. The thermostat itself is rather smaller than the one at my local accessory shop so this may be an interim engine that is largely undocumented. I removed the stat and refillled the engine which seemed to reduce the losses. For the first time, there seemed to be a trickle of warmth from the heater. Slackening the aforementioned pipe allowed a lot of air out. The pipe was new, as well. (A clue, perhaps?) The heater became hotter.
I took the car for a fifty mile run, working it very hard.
The following morning, Release cap slowly; a little pressure in system. The level was not readily visible but the filler neck refilled with half a litre. Turning the engine over caused water to issue from filler neck. Bleed end of rad. Level drops by half a litre.
This seems to be it. The level at which the top hose enters the rad is below the top of the rad itself. Even filling the engine gently will cause an air lock if the bleed is not slackened here. Having verified the feed to the coolant switch became live only when the ignition was on - (A damn silly idea if I may say so- I'll mod that out!)A new switch was fitted. By now, I am completely blase about bleeding the cooling system.
Final bleed. Heater fully on. Remove completely both bleed screws and disconnect the pipe at the rear of the engine. Fill using a strong hospipe jet and water issues from block. Plug block with thumb. Water issues from rubber pipe. Heater is now full. Marry pipes. Continue filling with hose. Radiator bleed stops bubbling. Start engine. Close top bleed screw and replace filler cap.
The engine has now no choice but to eject a little coolant in order to make up its expansion space. The heater will now warm up and a couple of blips will warm the radiator. Run the engine at high speed until the fan cuts in. If the fan cuts out again, the engine is in thermal equilibrium and if during the next fan cycle, there is no water coming out of the overflow hole, then you have probably cracked it.
This morning, I ran the car to work 20 miles and when I arrived, found no water staining from the overflow. There was a gentle hiss as the pressure bypassed the valve in the cap but that is just air expansion. All currently seems to be well. I shall fit a new thermostat this evening having checked to see that the level has not fallen greatly.
One further thing. I am getting a constant "low coolant" warning. I suspect that the clean water with which the system has been flushed has a rather low conductivity. I reckon that it will go away when I add some antifreeze.
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