It's an ever expanding minefield!
I would suggest calling somewhere such as NFU as they can go over care, custody and control etc with you and explin it fully.
I think it would be pretty risky to have a yard and just have public liability - even if its just a DIY yard.
I have Public Liability, a 'Product Liability' (covers me if clients sue me for damaging their horse, not getting it in foal, not training it properly, or selling them a duff horse - they may not have ANY legitimate claim at all but legal costs can hurt while you're winning your case if the other side has no money to pay your costs!!) And - of course - employer liability. All mine are through SEIB who are very helpful!
Companies such as SEIB do a livery yard insurance - last time we were a yard - about 3 years ago it cost £2k a year with a £1k excess. It ain't cheap, and the main reason that we closed.