I dunno what's going on with this. In the "Level Name" box in Level Properties I put in a level name which has 32 characters. But when I re-open the level I go into the properties to find that it has inserted the tileset filename. I click OK and get a message "Please enter less than 32 characters." Why does it do that?

It's poorly implemented code. JCS shouldn't let you enter more than 31 characters in the first place. Just use a level name that is 31 characters long at the most and you should be fine.

Thanks.
I have shortened the level name now.

It's because if you enter 32 characters theres no room for the null terminator and your level name will collide with the tileset filename (I think) that follows afterwards. I had a case like this in the past.