Onthe pilot.familysearch (free) site I've found a family in Hartford in Huntingdon whose children are being baptised in the 1820s. They are Thomas Roberts and Anna.
Joseph, baptised 30 November 1822, so his death certificate age is correct but his census age may not be
Elizabeth, baptised 6 August 1826
Rosanna, baptised 1 June 1828 - a name that is repeated in Joseph/Charlotte's family
Sorry that hasn't helped with the marriage record, but at least it gives you Joseph's parents and sisters if you didn't have them before.
Your last post - I agree with you. The biggest mistakes I've made were nearly always based on my roaring ahead following assumptions, and ones I've got right are usually painstaking step by step.
Looking at the 1851 census, the Charlotte I found baptised in 1827 is the only one coming up in London of the right age, living with her parents John and Charlotte. She is married and listed as Charlotte Jones, with a daughter age 1 month named Sarah Anne.
This gives a few possibilities, the first of which is that this may be a giant red herring, and not related to your family at all.
The second is that Charlotte married and had a little girl then was widowed, or maybe even the little girl died.
Would it be worth trying to see if there are any records of a marriage between Joseph Roberts and Charlotte Jones? If it was a shortlived marriage it's possible and Charlotte's maiden name, for all the children's birth records, would still have been Mead.
- added - Good theory, but I've just done a quick search and a Joseph Roberts to Charlotte Jones marriage isn't coming up either. Sorry