Ah, I've found a workaround.
It seems the strange lines were carried over from the top of the circle tiles.
So I've changed my tiles like so - note the black first line on the "top" tiles:
Firstly - great site for a great game!
I've downloaded Tile Layer Pro and edited the star as shown in the tutorial.
However, I am getting two strange lines below the circle. Any ideas?
Any help appreciated!
matt