After receiving a very unhelpful answer, a friend found an answer online.
I spent several weeks having to crawl in through the back door then insert a flat screwdriver in a small aperture in the lock body, this is covered by a clear piece of tape, a white lever can be seen, insert the screwdriver to the right of the lever then push it to the left,, this will then release the tailgate.
After checking various things I found with the tailgate up and replicated the locking motion by using the shaft of a screwdriver, the lock operated and could be released with the remote, but if I closed the tailgate it wouldn't unlock.
This was traced by an electrician to a broken wire that made contact with the tailgate up, but didn't make contact when the tailgate was down.
This must be the worst car I can remember in my 60+ years of driving for rubbish electrics, pity it isn't as reliable as the rest of the car.
Finally why not a plain simple lock and key instead of clever electrics that fail.
To the bored person who answered very unhelpfully.
Now that wasn't so hard to do.
