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There are no Charter baptisms in Bluntisham, which is not a good start.
I can see William, Sarah and presumably daughter Mary aged 1 living in March with his Beckerton in laws in 1841 but I can't see him in 1851. Have you found him in 1851, and where did the information come from that he was born in Bluntisham?
A John Charter from Bluntisham married Elizabeth Hawkes at Holywell in 1814. There were quite a few Charter marriages in Bluntisham at around this time, which makes the lack of baptisms curious. Were they all non-conformist?
David