Equine legal advice needed please

I have a horse on a grass livery site in England.... No contract has ever been put in place. My horse has lost condition in her time there even though she has been receiving multiple feeds daily and had been wormed but keeps getting reinfected (I think due to the yard owners ignorance) so for her own welfare I gave seven days notice after that I'd be moving off. I said I'd happily pay for hay(119 pound) for the month she was there even though they where rarely given it and if the horses where hayed it was never enough for the 15+ horses on the site all who are underweight. Due to the urgency of my moving I offered one of my saddles (valued at £350)to settle and hay debt straight away or she would have to wait and receive it on the Tuesday before I moved off. She denied this and stated I must pay for another month livery as notice and then the hay on top of that...... A few days later she then stated I need to either pay today or my horse would be left on the front for me to take, I said I would be coming up after work to take it she then back tracks and says they are now not allowed to leave until the full debt is paid and the month is honoured and she has changed the code to the gate so I can no longer access my horse who needs multiple feeds a day and desperately needs moving for her own welfare I don't understand how she can do this with no written legal or verbal agreement?

What can I do?