Well I thought that i'd fixed the problem, but hadn't - it returned.
I do now believe however, that it is finally sorted.
I had used the original regulator box, having removed the electromechanical device, soldering the wires of the new regulator to the terminals. herein was the problem. The terminals are made from two parts, a pushed in bush into which the wires of the harness are pushed and the terminal which is placed over the other end which was then riveted. The result was an intermittent contact. When I soldered the wires into the bush, I resolved the problem.
This tool me ages to solve, but I thought I'd let you all know so that no one else suffers from this malaise!