In addition to this post, is it possible that I have connected up the radiator hoses incorrectly. I would have thought that this would somehow 'break' the system by making it work in reverse but maybe not. I am starting to think that the two hoses attached under the radiator cap are the wrong way round..... I currently have the thicker one (from the manifold?) attached at the top and the smaller one which leads to the overflow bottle attached below this.

I would feel pretty stupid if this was the case but also glad to find the fault.