Splinter,
As no one else has come up with a reply. I'll try and help - I'm no expert.
I looked on the IGI for Samuel Heath christened 28 Feb 1758 Bovey Tracey, Devon, England - father Charles Heath - yes I found him.
I don't thing this Samuel Heath married Mary Tapper 18 June 1773 - he would have only been 15 years old at that time.
Also if Johanna born 2 July 1799, was the first born child she was born 26 years after their marriage.
Think you will have to find another spouse for Samuel Heath.
Good luck
Sarra