Problem fixed! The bizare symptoms fooled me into thinking it was obscure problem. It just proves that you you should check the simple things first. The fault?.....oxidised fuse contacts. Fuse cleaned, problem solved.

The tip that the 2nd door plunger switches are there to enable the windows to be operated with the ignition off was very helpful. The fact is that this wasn't the case on my car....got me thinking. Explanation...the central locking / electric windows circuit is served by fuse 12 with the ignition off, but has a direct 12V feed via the ignition switch when the ignition is switched on.
Thanks , Pete