WBF, what exactly do you mean - how open is open?
Do you have any fruit netting, greenhouse shading, landscape fabric, tarpaulins (might need to add tent pegs, we did anyway), sheets of plastic...? Do you have any outbuildings?
Yes, the fine is up to £5,000, btw, and up to 3 months in prison, and that's per offence. It's on Defra's website.
I do hope we don't get an outbreak, we live somewhere with loads of tresspassers who completely ignore the 'private - no public right of way' signs and are annoying enough as it is.