I think your problem will be that the valuation must be INDEPENDANT. So if your OH (or their solicitor) don't like the figure you get then they'll get their own valuer out. I doubt I'm making you feel much better. But you'll survive, honest you will.
I'd like to know how she's going to prove it was a kick from your horse that hurt her back. And not her mucking out. Or pushing a wheelbarrow. Or riding a horse. I'm sure years of that kinda thing have caused a bit of wear and tear on her spine.......
Hands up all horsey folk who DONT have a...