Farmers must keep records of all pesticide applications. There is currently no obligation to disclose farm spray records to the public, but it is considered good practice to do so.
The National Farmers Union has published a Spray Operator Guide which recommends being open with the public.
Local councils will usually provide information on what has been sprayed, though it can take some time to find out, as spraying is often contracted out. If the council is unwilling to provide this information you could try making a request under the Freedom of Information Act.