Hi Derek,
That's the first I have heard of any B18/B20 engines not having that tube in the head.
The cooling system seems to be working OK now. I did swap the water pump for peace of mind. I then filled the system as carefully as I could and went for a run. I only filled the reservoir bottle to a little over the minimum line. At the end of the run the level in the bottle had risen a couple of inches and the radiator was hot top and bottom. So I would cautiously say that all seems to be OK now.
Thanks everyone,
Andrew
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