Basically there are several things to consider here, like:
How long is a piece of string?
How much are people in your area prepared to pay?
How big is the field?
How good is your actual land - is it flat, fertile and well fenced or rough and unfenced?
How much is your bottom line for having someone else on your land and potentially making a mess?
£20/acre/month, ie £240/year is reasonable for good land where you would get that much in annual rent from an arable farmer, providing it's a reasonable sized field.
If you were to let it for horses and it was well fenced you could get £15/acre/week.
I pay about £120/acre/year for cattle/sheep grazing, with no water and I repair the fences. I wouldn't pay £240/year unless it was something really special as farming isn't that profitable.