Boulby Barns Farm is for sale, along with its sound mirror. Offers in the region of £695 000.

Within the paddock is a Grade II listed Listening Post dating from around 1916 and used to detect approaching enemy Zeppelins and other aircraft. This structure is believed to be one of only 3 remaining in the country.
Source: Smiths Gore/Rightmove

Presumably this refers to the similar mirrors at Sunderland and Selsey.

Thanks to John McMillan for alerting me.