Diagrams proved invaluable, in diagnosing the fault. The problem was faulty electronics in the new mid-point valve. All now fixed.
Thanks very much for the help.
My central heating failed a couple of weeks ago, and the problem was an ACL mid-position valve (MA1/679) leaking water up the spindle into the electronics. The valve has now been replaced, but has introduced a strange problem.
The mid-position valve gets stuck in the mid-position under...