Hello David,
I have had this happen twice with my Mark 1, which is probably similar. The issue occured when the catch mechanism, on the bottom side, flipped toward the front of the car, rather than towards the back.
I was able to open it both times by inserting a long slender screwdriver into the lock area through the gap, and twiddling against the mechanism. This while applying some up and downward force to the bootlid.
It was a frustrating ordeal on both occasions, but it finally let loose!