Does the field belong to the Parish Council, or are they acting as custodian trustee for another charity? Confusingly, a custodian trustee is not a trustee in the charity law sense. If you're not sure, the easiest way to find out is to do an online Land Registry Map Enquiry. The Title Register will cost you £3.00, assuming the land is registered and will confirm the beneficial owner.
If the Parish Council is merely a custodian trustee, it acts on the instructions of a third party, so needs no indemnity insurance. If the Council is acting as Trustee in its own right (a managing trustee), indemnity insurance is advisable, unless the charity's governing document specifically prohibits this, as some older documents do. Indemnity insurance is normally included in most charity insurance policies these days.
I've suggested the Land Registry route, as it is more reliable than asking someone who has been on the Council for years, as the all-important difference between managing trustees and custodian trustees is often lost in the mists of time and the roles become confused. I am dealing with two such cases at the moment, where community centres have been built by Parish Councils on land they thought they owned, but have now discovered that they don't.