Well I do all my editing with notepad. I copied that desc.ext from this deathmatch tutorial I downloaded from here a few moons ago. I even wrote my own desc.ext with notepad and it did the same bloody thing.
... so I finally took a description.ext from an old deathmatch scenario I made on versus isle and stuck it in place of the offending one. No problem... weird. That's all I can say. Because the one I replaced and the one I replaced it with were exactly the same... at least as far as I could see.
Anyhow, my mission has respawn now, thankfully. Thanks for your help guys.