During winter barrow farm is usually open most weekends depending on weather forecast, you just have to get in touch so that they are aware you are want to go then you simply pay a fee for the day riding, if you wish to stay the night you pay like an extra fiver and then a further normal day fee for the next day if you decide to ride again simples. As for the barn i cannot not give you any information but also you might want to know that during the summer holiday they are open on tuesdays and thursday for a set time but apart from the summer holidays its normally just weekends. When i go there it usually cost me around 20-25 so not very dear at all. I strongly recommend having a look at their site and joining the facebook chat page.
Hope this helped.