FIXED IT!
Ok, so I still have stuff to fix on the car, but this problem has been sorted!
It was the microswitch assembly that registers the roof is fully closed, the one at the top of the windscreen. I replaced that as I had (still have, just different now) roof issues. I replaced the latch, and bingo, I am now able to lock the car!
Feeling very pleased with myself
Now all I have to do is work out what I've done wrong putting all the roof back together.....
