Having looked a the Charlotte Lester witness signature from 1804 and 1839, I wouldn't like to say that is the same person. Yes there are a few similarities, but there are also differences which could be caused by a large difference in time, but there is no proof it is the same person.
You say you aren't aware of any other reputed children to George and Elizabeth, yet you mention the ancestry tree which has the George and Elizabeth Saltmarsh marriage. That same tree has other children listed to George and Elizabeth, including others listed as baptised in Barking - Jane (1814), Elizabeth Anne (1819) and Charlotte (1820). Those are the baptisms that Trish has already mentioned.
The ancestry tree also lists a Henry George who is supposed to have been born in Whitechapel in 1807, although no baptism is listed. The tree does have a marriage for a Henry George Lester to Mary Ann Maria Knight on 7 April 1839 at Poplar All Saints. The marriage is viewable on ancestry, he is listed as a mast maker, father George a cordwainer. One of the witnesses to that marriage is John Samuel Lester and the signature looks the same as for your John Samuel's marriage. The Charlotte who witnessed John Samuel's 1839 marriage could well be his sister.
Henry George and Mary Ann Maria would appear to be in Kensington on the 1851 Census as Leister, he is listed as born Whitechapel aged 44 years and with them is his father George a widower, formerly cordwainer, aged 78 born what looks like Bow City, Middlesex. This would tend to tie in with the 1804 marriage in Whitechapel, although I can't see a baptism for Henry George nor his father.
There is an Elizabeth Sir Lester death registered Dec qtr 1844 Romford Registration District, but whether that is George's wife can only be confirmed from the death certificate. Romford Registration District covers a number of places. Ancestry is showing a burial of an Elizabeth Sir Lester in 1844 at Barking, the source is familysearch. Familysearch are also showing an Elizabeth Saltmarsh baptised at Romford in 1777, which could possibly be her but you'd need to check through the Romford parish registers to see if that child died as an infant or possible married there.
There is a marriage of a Charlotte Lester in Sept qtr 1845 Kensington Registration District, unfortunately not showing on ancestry. One of the possible spouses is a John Gibbs. In 1851 there is a John and Charlotte Gibbs in Stepney, she is listed as aged 30 born Barking and they are living in Mile End. However, you would have to purchase a copy of the marriage certificate for that Charlotte Lester to confirm if she did marry John Gibbs and her father was George Lester. Its a shame the marriage doesn't appear on ancestry because the signature could then be checked against the 1839 witness signature. Its worth bearing in mind though that one of the other potential spouses of John Gibbs in 1845 is a Charlotte Amy Wood. That said, there is a baptism in Stepney in 1848 of an Elizabeth Sir Bedon Gibbs to John and Charlotte, a merchant clerk from Limehouse. Not quite the same profession as on the 1851 Census (and the child would appear to have died by then) but interesting middle name in light of the 1844 death in Romford RD.
