Willingham parish register is on the IGI as a controlled extract, so is pretty reliable. There is only one Phoebe christened 1819/20 in Willingham - Phoebe Gadsby christened 7 Feb 1820, twin daughter of William and Mary (her twin was Rebecca - per the Cambs FHS baptism index
http://www.cfhs.org.uk/Search.html). Whilst it is pure supposition, this may be the Phoebe who married John Wesson, but on the other hand it might be a complete red herring!
Judging by the ages of the children in the 1861 census - per your earlier thread - they could have married 1847-49. A search of the GRO indexes at your local LDS or main library might therefore show their marriage entry. I can't see anything on FreeBMD, but their marriage transcription rate for 1848/9 varies between 30-95% and has hardly been started for 1847, so the marriage might not have been transcribed yet.
I've had a look at the 1871 entry for Phoebe and I think her age is probably 51 rather than 57 (the figure is bad, but looks more like the enumerator's 1 rather than his 7), which would be consistent with 1841/61/81.
I think the marriage that Mike has found is the other John and Phoebe Wesson who were living in Cambridge in 1841 and 1851 and who were both born in Somersham.
Hope this helps
Regards
David