Dear Stewart,
The website: http://stmarys.thornburyroots.co.uk/earlier%20vicars.htm states that all three sons of the Rev Thomas Willis died young.
"Thomas Willis was Vicar of Thornbury 1728-1748. He was grandson to William Beaw, Lord Bishop of Llandaff. He married Ann Walker the granddaughter of Thomas Lushington of Sittingbourne in Kent and the daughter of George Walker and his wife Ann (nee Lushington). Thomas and Ann Willis had three sons. Sadly they all died young. We know of the burials of two of them. Thomas Willis junior was buried 13th May 1730. A second son called Thomas Willis was buried 25th January 1736. A memorial stone inside thechurch was recorded by Bigland and it is still visible today."
However, the site does not document the death of the third son.
Kind regards,
Bob