After a bit of investigation (pouring water on to specific areas with paper towels inside the boot!) I managed to trace it to the water running in around the boot key area. You can actually see the water draining around the key hole
I've not removed the lock assembly yet.
Has anyone else had this problem? If so, what was the fix?
BTW Car is only 3 months old but MMC is too far to take it back for a fix.