The window control module in the door has two connection sockets - one is fed from the switch -the other feeds to the window raising mechanism. My problem was on the switch side connector.
Removing that plug accesses a multi pin connection (maybe about 20 pins and sockets inside the main plug). I noticed that one of those pins was blackened - I supposed with sparking - that pin turned out to be the permanent live. On VERY close inspection I noticed that one of the nearby sockets had corrosion inside it (I supposed that to be the earth return for the control). Bear in mind I had "examined" this connection several times previously and missed this issue - the sockets are tiny. I managed to carefully clean inside the socket with the smallest instrument screwdriver I had.
This solved the problems I'd been having for months - intermittent window failure, frequent need to do the reset procedure and finally total failure. It has been several months since I did this and (touch wood) no problem since.