You can book a pitch (with or without an electric hookup) by going to https://www.heatonhousefarm.co.uk/accommodation/ and click "Book your stay".
A pitch is priced at £10 per adult, £5 per child, and an electric hookup is £15 per night. You also have the option to add breakfast served by Stable Yard Catering between 9am and 10am.