So… interesting.
I stripped the door down, fitted the new lock module and tried the lock. Bingo it started working.
I tested, opening and closing the door and it was fine. Result. I tried again later thinking it would be fine and there was a problem, it wouldn’t lock again.
At that point I gave the wiring between the body and the door a squeeze and tried again. It stated working. I unclipped the rubber boot and inspected the wiring, expecting to find a break but saw nothing untoward. So I sprayed electrical contact cleaner into the pins and refitted. Seems to be working again, for now at least.
I found somebody on YouTube who had similar problems in his door and contact cleaner worked for him. Time will tell if that was the problem and if the fix lasts.
For those of you in a similar situation, it may be worth checking the wiring and even using contact cleaner on the pins, before stripping down. Much cheaper and can be done in about five minutes.