Janet10 on Family Tree Circles

My 3x great grandfather is David Appleton, born in Devonport 1794
He married Mary Ann Munyard in Deptford, Kent, UK and appears to have been a mariner with the Royal Navy.
When he retired he worked for Somerset House as a clerk for the Navy at Devonport.
He and Mary had at least 5 children - one has Hockin as a middle name.
George became a doctor in London and worked in Cornwall in Helston and then Landewednack on the Lizard, Cornwall.
He first married Emma Tredwin, she died 2 years after their marriage (cause unknown)
He then married Martha Maria Ellis from Helston, where her parents ran (I believe the name was The Dukes Head Inn) an inn in Meneague Street (now owned by Betfred!!) and all I have gleaned is that there was a fire.
George is my 2 x grandfather and whist I can trace most of my family from David and Mary Ann, I simply can't trace the names of David's parents and where he originated from, even though he was apparently born in Devonport.
It was a time of battles at sea and I almost assumed that maybe he lost his father at sea but I don't know his name for sure.
I think I found a record of a John Grigg Appleton (?) staying at Penmennor Hotel at Lizard Point, next door to the home George and his family lived. I think a relative? Big coincidence...
If anyone can help in any way I would be most grateful.
Regards
Janet (nee Appleton)