I have mine with NFU Mutual, costs £6 a month, and that's cover for personal injury and third paty liability. Doesn't cover me for riding my own horse though.
I have NFU Mutual, its great, covers me for any horse in my care but not owned by me. There's £1000 vet fees and owners tack cover. Costs between £56 - £63 per year. I had a bad fall last August and they paid out quite quickly and were very helpful.
If you decide to go for NFU I can give you a telephone number etc - PM me for further details.