No idea whether question is still of interest, but the the Martell family of Fawley is one I researched quite a bit a few decades ago.
There's a possible marriage on Findmypast for a John Stephens and Ann Martell in Dec 1809 (day apparently not determinable) at St Mary's Newington, Surrey, which is only two miles from Covent Garden. That could be right couple, but it's a densely populated area and John, Ann and Stephens are very common names.
There's a 23 Dec 1787 baptism for an Ann, daughter of John and Frances Martell, in the Fawley parish register. This matches Ann's age and has the correct mother's name, and is the only baptism of an Ann Martell with a mother's name recorded as Frances anywhere in the county around this time. At this time, Exbury was in Fawley parish, which matches her place of birth on the 1851 census. There's a Fanny daughter of John and Fanny Martell baptised 24 May 1789 in Fawley, but these seem to be John and Frances's only recorded children. It's entirely possible that the parents were the John Martin [sic] and Frances Carpenter married at Fawley on 30 Oct 1786 – the two surnames do get confused.
Fanny Martell was buried 9 Dec 1853 at Fawley, aged 86 and described as being of Exbury, putting her birth around 1767. The 1851 census puts it a year earlier and tells us she was from Beaulieu. This matches the baptism there on 30 Apr 1768 of a Fany [sic] daughter of John Carpenter. She is the fourth in a sequence of seven children of John baptised in Beaulieu: Samuel (10 Jun 1761), Hannah (17 Feb 1763), John (31 Mar 1765), Fanny, Milly (27 Jul 1770), William (20 May 1773) and Elizabeth (23 Aug 1775). Samuel's father is named John junior, while Elizabeth's mother is identified as Hannah. A John Carpenter married Hannah Wyatt at Beaulieu on 24 May 1760.
It seems pretty likely that John Carpenter was buried on 28 Mar 1809 and Hannah on 27 Jun 1811, both aged 80, putting their births in about 1729 and 1731, respectively. This matches a Beaulieu baptism on 30 Jan 1728/9 for John, the second of eight children of John and Mary Carpenter baptised there between 1728 and 1737. A John Carpenter married a Mary Togood at Beaulieu on 25 Mar 1722, and Mary is likely the same person who was baptised there, daughter of John Togood on 26 Oct 1696. I cannot immediately see baptism for the older John Carpenter nor for Hannah Wyatt.
The only plausible burial I can find for John Martell is on 19 May 1820 at Fawley, aged 57, putting his birth in around 1763. There were plenty of Martells in Fawley and even specifically Exbury around the time – they were my ancestors – but the first name 'John' is conspicuously absent from the family at this time. If John was indeed from the Exbury/Fawley family, rather than someone unrelated who had moved into the area, perhaps the most likely possibility is that he was a child of the William Martell of Exbury who had children baptised in Fawley in 1756 (twins) and 1760, but it's unclear who this William is. Perhaps he's the William Martill who married Olive Barnet on 28 Dec 1740 at Fawley, in which case there were presumably other earlier children and a child in 1763 is just about possible. Olive was buried at Fawley on 10 Oct 1773, and William was most likely buried at Fawley on 20 Feb 1780. Neither has an age listed. (The William buried on 10 Jan 1783 is most likely the son of Jacob and Sarah who was born in 1782 as they they had another child with this name in 1786.)
According to the transcripts made by the HGS (which serve the basis of the data on Findmypast), there are two children of a William and Olive Martin baptised at Fawley in 1744 and 1746, and further children of a William Martin baptised in 1748, 1750 and 1752. It seems pretty likely to me that this is the same family – the 1756 and 1760 baptisms fit neatly on the end of this sequence, and there are no burials for a William or Olive Martin. I've seen other confusions between the Martin and Martell surnames in Fawley at this time.
The other Martell family in Fawley parish at about the same time as William and Olive was Jeremiah and Joanna. They have four children in the Fawley baptism register (three as Martell, one as Martin), but from Jeremiah's will, we know they had at least eight. Two of the missing children are Jacob (b c1750) and Philip Benjamin (b c1740) who later had families in Fawley. Philip Benjamin may have been baptised in Lymington in 1736 – the HGS index transcribes it as two sons of Jeremiah Martin baptised on the same day, but perhaps it was really one with a middle name. The parents, Jeremiah Martell and Joanna Grossmith, married at St Michael's Southampton on 21 Nov 1734, and from her unusual surname, we can be fairly sure she must have baptised in 1712 in Wickham (which is midway between Fareham and Bishop's Waltham). But I can find no plausible baptisms for Jeremiah, either as Martell or Martin, anywhere in the mainland Hampshire. I think it's quite plausible that William and Jeremiah were brothers, and perhaps the Isle of Wight would be the obvious next place to search for their baptisms and perhaps those of the missing children of Jeremiah and Joanna. It may even shed light on the origin of John Martell.
