Hello Tiemen,
I've just done a bit more investigation into the Yarmouth connection, and think that Harleston, Norfolk, as John Cobbold's place of
birth is still a possibility. But you may need to look somewhere else for his place of
baptism. For example, John's mother may have gone to her home parish to give birth in order to have the help and experience of her mother, but she would afterwards return to where she was living with her husband, and the child would be baptised there.
I've also found evidence of John and Fanny being in Yarmouth in November 1743. The original Yarmouth St Nicholas parish records are on familysearch and there I can see the baptism of "John Corbell s[on] of J[oh]n & Frances" for 24 November 1743. See
https://familysearch.org/pal:/MM9.3.1/TH-266-12350-5913-81?cc=1416598&wc=661779 . Corbell is OK as a variation of Cobbold.
Then I did a standard search in familysearch for children of John and Frances 'Corbell' born in Yarmouth, Norfolk in 1745+/- 5, and immediately below the record for little John's baptism I saw one for the burial of a John 'Corbelling' son of John and Frances, in Topcroft, Norfolk, 1745. Corbelling is not a surname I have come across so I was a bit suspicious. I went to the original Topcroft parish records on familysearch
https://familysearch.org/pal:/MM9.3.1/TH-267-12127-86127-12?cc=1416598&wc=660907. I was right! The entry reads 'John, son of John & Frances Corbell
was [that's what the transcriber mistook for 'ing'] buried on the 27th day of December'.
I haven't looked any further in the Yarmouth or Topcroft parish registers, but it might be worth spending some time looking there for either John and Frances' marriage or the baptisms of their children during the 1740s. You might be lucky and find a clue to John's place of birth. And maybe also try registers for other parishes in the area.
If you look at a map for Topcroft, Norfolk, you will find that it is about 10 km due north of Harleston-cum-Redenhall. And Harleston is so close to the Norfolk/Suffolk border that it could easily have been thought to be in Suffolk either by JC himself or by the person recording the Old Bailey proceedings.
By the way, you can see which original Norfolk parish registers are on familysearch here:
https://familysearch.org/pal:/MM9.3.1/TH-267-12127-86127-12?cc=1416598&wc=660907#uri=https%3A%2F%2Fapi.familysearch.org%2Frecords%2Fcollection%2F1416598%2FwaypointsBut perhaps you know that already!
Have you posted your inquiry on rootschat's Norfolk board? It might be worth doing that, and ask for someone to check the Harleston-cum-Redenhall registers for JC's baptism. That would be a start!
Birtle
By the way, I'm curious. What's your evidence for your John Cobbold living in Ipswich in 1745? Are you sure it's him and not one from a different branch of Cobbolds? There were lots of them in Suffolk at this time.