In 1722 William Cayley went to Lisbon as secretary to Sir Thomas Saunderson, the British minister there.
[2] There is a collection of his diplomatic correspondence from 1735 onwards in the British Library, a number of letters reporting to the Duke of Newcastle and others, including Admiral Nicholas Haddock, on ship movements and other matters of interest to the British government, particularly causes of friction between Britain and Spain.
[3] In 1746 he sought, and obtained, permission to resign his post in Faro and return to England.
In 1754 he applied to the Duke of Newcastle for a government appointment in recognition of his work in Spain and Portugal.
He left Scampton to Richard Stonhewer with remainder to his cousins, the Cayleys of Brompton.