In reply to Mr G: I lead climbing trips (SPA) from my school and only ever used the LEA insurance. As long as your LEA has a sizeable chunk (say £5M plus) of professional liability for its staff working on school trips, that's all you need.
If you want to run trips as a freelance for clients outside of your own school, you'll need independant insurance; the MLTA scheme is the way to go. I pay £180-odd for SPA only.
Note that you can't run trips for children, though, if they're not members of your school, unless you have an AALA licence. This annoys me no end.