Hey I just thought I'd give an update on this incase anyone else has this issue as I believe I've got to the bottom of it. So just to be thorough I changed every texture name to exclude any special characters at all, I did this in both the texture folder and the .dat file to make sure they matched. I did the same with the tiles/custom modifiers .dat file just to be sure (these where the only 2 things that I had added before the error)
However I was still getting the error so I decided to start deleting one texture at a time and load the game and after about 4 texture the map loaded fine. I re-done this with random textures just incase it was one specific texture that was giving me issues but no matter the texture after about 4 got deleted the map loaded so I believe that I actually reached the texture limit for a map.
Atleast now I know there is a limit and I can reduce my scope of my levels
Appreciate the help with this Ragnar