Finally pulled my finger out and sorted this out - the spoiler hard up against the bootlid and at a weird angle rather than flush with the lines of the bootlid and bumper.
At first I assumed it was something amiss with the retracting mechanism, but soon spotted that it didn't seem to be mounted at the rear - it was just sitting on top of it as you can see in the below pic.
So I drill out the three plastic rivets holding the front edge
I re-mount it by clipping it onto the rear edge first, then re-rivet it at the front, using standard metal pop-rivets for now.
Voila! Thoroughly satisfying
