Actually they did fit perfectly the only issue I had was they where a little difficult to get hold of the ones in the middle to pull them out of the tray.
So my thinking cap came out, I needed something to stop the Markers going to the very back of the tray so that I had a little bit extra to grab hold of.
My answer came from the pound shop
Draft excluder tape, self adhesive and high density cut into little strips and stuck at the back of each compartment, works beautifully.
All neat and tidy and easy to get out.
You can see the excluder tape in each compartment in this picture.