Hi Pauline,
I've got a family tree which says John Jelf born 1794 in St George, Bristol. He was the Son of Moses Jelf and Mary Williams. He married Mary West on 25th Aug 1820 in St. George Bristol. In 1841 John was a labourer and he and Mary have 6 children the eldest of which is Thomas Jelf.
The only information I have on Moses, John's Father is that he was born in 1758 and married Mary Williams on 5th Sept 1784. He died 27th Oct 1815 aged 57 years. Unfortunately I haven't been able to get back further than Moses and Mary.
Hope that helps, let me know if I can answer any more questions!