Hi Claire, yours is an interesting puzzle – but I’m frustrated by not having access to census records! Especially the 1851 census.
Of the known members of the ROBINSON family (presumed children of Joseph ROBINSON and Mary GUEST), the following have been found in
1851 as far as I am aware – Anne (b Rotherham 1805), Joseph (b Gainsborough ca 1816) and Ellen (b Gainsborough ca 1831).
Thomas (b Gainsborough ca 1813) probably died in 1850.
Still missing in 1851 are (at least) William (b Gainsborough ca 1818 – what are taken to be William’s wife Rebecca and daughter Clara are living with Anne and Ellen), and Frederick George (b Gainsborough ca 1821).
And Fred’s daughter Dorothy (b Gainsborough ca 1845) – unless, of course, she has already been found under another name.
It would be particularly good to find in the 1851 census the bride and groom from that tantalising 1849 marriage in Doncaster of a Frederick George ROBINSON with Sarah Ann BERRY the only other name on the page in FreeBMD. If they could be found (anywhere!) it would be a cheaper method of getting more information than buying the marriage certificate - which might be a last resort.
Searching might need to be creative – I recall that katherinem found Frederick G. ROBINSON in 1871 but mentioned that, in Ancestry, his name had been transcribed as ROBERSON.
I am in Australia so my mind automatically goes to Internet sources and to films of church registers which can be ordered in to LDS FHCs - which is why I mentioned that possibility.
So Claire, if you are in a position to look at Gainsborough church registers yourself, or if someone else can do so for you, that will be great but, if not, perhaps the LDS films would be worth pursuing.
For Gainsborough in the relevant time period, the FHL Library seems to have not a bad coverage (there are also earlier and later ones) - including the following:
* Parish registers for All Saints Church, Gainsborough, 1564-1948 / Church of England. All Saints Church (Gainsborough, Lincolnshire)
* Parish registers, 1844-1973 / Church of England. Holy Trinity Church (Gainsborough, Lincolnshire)
* Church records for Gainsborough and Gainsborough Circuit, 1838-1968 / Wesleyan Methodist Church (Bridge Road, Gainsborough, Lincolnshire)
Includes entries for places outside of Gainsborough that are in the Gainsborough Circuit, including, Torksey, Willoughton, Upton, Blyton, Knaith, Marton, East Stockwith, Stow, Sturton, Saundby, Misterton, Sturgate, Willingham, Worksop, Kexby, Retford, Somerby, Brampton, Heapham, Bole, West Burton, Walkeringham, Scotter, Saxilby
* Church records for Primitive Methodist Church, Gainsborough Circuit, 1843-1935 / Primitive Methodist Church. Gainsborough Circuit (Lincolnshire). (Baptisms 1843-1945)
Includes entries for places outside of Gainsborough that are in the Gainsborough Circuit, including, Sturgate, East Stockwith, Misterton, Kirton in Lindsey, Gringley on the Hill (Nottingham), Beckingham (Nottingham), Heapham, Springthorpe, Somerby, Marton, Morton, Kexby, Torksey, West Stockwith (Nottingham), Birmingham (Warwick), Retford, Hull (York), Walkeringham (Nottingham), Corringham, Willoughton, Sheffield (York), Bardney, Brampton, Upton, Horncastle, Aisby, Pelham, Leeds (York), London, Lincoln (city), Heckdyke, Willingham, Ingleby, Haxey, Saundby, Hartlepool (Durham), Grantham, Ollerton (Nottingham), North Wheatley (Nottingham), and Scunthorpe.
* Church records for Wesleyan-Methodist Chapel, Misterton, 1844-1968 / Wesleyan Methodist Chapel (Misterton, Nottinghamshire).
* Record of members, 1843-1922 / Church of Jesus Christ of Latter-day Saints. Sheffield Conference (Yorkshire)
Hi Valda, I guess we’ll have to agree to have our own interpretations of the application of Occam’s Razor in this case

I’m keeping an open mind.
I take it that Geoff E was using more than one source for baptisms.
In one post Geoff referred to a
baptism register saying “I coudn't see Dot in the baptism register ... Ellen (dau of Wm and Ellen HOSTHEAD) was born and bap 29th Jan” (he didn’t spell out that this was in 1846 i.e. was the Ellen who is recorded on FreeBMD in Mar qtr 1846 as HOSTKEAD; I note that there is also an Ellen HOSTHEAD – perhaps also a dau of Wm & Ellen? – born Mar qtr 1840 and death of an Eleanor HOSTHEAD Mar qtr 1842).
In a later post, referring to Frances Elizabeth HOSTHEAD (bap 1823), he referred to the
BIVRI2 saying “Her baptism does not appear on the IGI but is on the BIVRI2 disks. I can’t find a marriage for her.”
Just for completeness (though, of course, the HOSTHEADs might be a complete red herring) ... You indicated earlier that Jane Popple (bap 1827), Susanna (bap 1828), William (bap 1830), and Henry (bap 1835) do appear in the IGI (which goes to 1835 though it doesn't seem to include Frances Elizabeth bap 1823).
Geoff mentioned that William & Ellen had several children (he didn’t name them) including two with middle name Pepple/Popple (name of the other one?) so presumably there was at least one we don't know about. We also know from the 1851 census about Elijah (his birth as Elijah Philipson HOSTHEAD Mar qtr 1838 is on FreeBMD) and Ellen (her birth being recorded as HOSTKEAD Mar qtr 1846 on FreeBMD).
Regards,
JAP