Thanks squeeb! That did it.
Although it does work, due to the number of tiles (my scene is huge, 300 x 300 tiles) there's a lot of lag when viewing the map. Not a huge problem for gameplay as the game is paused when the map is up, but I'm still going to have to switch from using drawing blocks to using the Image API.It's drawing around 45,000 shapes!
Ahh.. i see what youve done.. sweet... it can be optimized