SpudFiles

Thanks Clide, I'll would see if that works, but I can't be sure your way would work with British pipe ( I'm not sure the diameter would be right) and I don't have any 2" lying around to try it. Darn.

You can do anything with a dremel!
I like to clamp my dremel to my workbench, and put in basically any bit that would cut straight in (like the ball-end one, or a pointy one, or the one that looks like a drill bit but cuts sideways). Put that right next to a block of wood clamped down. Then, clamp another block of wood (or whatever) at whatever length you want the O-ring to be cut.
That sounds confusing, heres a diagram:

Another good idea, I'll see if I can rig something up with my workmate