It didn't work I'm afraid.
My ISP got back to me last night. They do indeed shape traffic but for which port range I don't know. It seems like quite a broad range however. They gave me a couple of ports to try which they have specifically opened up for LFS; I tried these with our server and they work absolutely fine (I've set them back now) - it's the fastest i've ever seen LFS connect to anything, and this was bang in the middle of peak-time.
The problem is, it doesn't mean anything unless other servers use these ports aswell, or the port range for this is standardized so that ISP's can open up this specific range. If not, I'm going to have to look at changing my ISP so that I can play this game
