Was he a Quaker? There is a Robert Fothergill buried Bristol 1788 aged 80, a Quaker, who could have been his father. On the Pucklechurch marriage Robert's parish is shown as Keynsham, which is the outskirts of Bristol. I would have thought Mallerstang was very unlikely. I tried to post the Quaker burial details from Find my past but was unsuccessful, it says too many characters!
"England, Bristol, Non-Conformist Church Records, 1777-1936," database with images, FamilySearch(https://familysearch.org/ark:/61903/1:1:Q2Z7-5WBH : 11 March 2018), Robert Fothergill, 17 May 1788; citing Burial, Bristol, Gloucestershire, England, United Kingdom, Bristol City Archives, England; FHL microfilm 004320050.
||21 Mar 1779|
||Pucklechurch, St Thomas a Becket|
|Spouse's first name(s)
|Spouse's last name
|By licence or by banns