I used to keep my horse on livery in that area years ago but my knowledge is out of date. There is (I think it's still there!) a saddlery in Montacute called Stax - why don't you ask in there?
I'm at Yeovil Marsh, really nice yard, hacking isn't perfect if your horse isn't good in traffic (have a 50 yard stretch of the A37 to get along the verge of, but once you are in the village it's great) we do have a 30 x 60 ish school which is kept really nicely, plus feilds we can use to go for a blast round.
There are a few others but further away from Yeovil.