I'm having some trouble linking people into families...

My 4th great-grandfather (James Henderson) was born according to the England Census in 1811 in St Martin-in-the-Fields. Through his marriage certificate, I know his father's name was James as well.

However, there is no James Henderson born in 1811 in St Martin's... There is one in the same parish, but in 1813, and there is one in 1811 but baptized in St James Westminster (basically the same area). Both children of a James Henderson and his wife Ann.
When I do further research, there are a bunch of children born in St James Westminster, all children of James and Ann Henderson. The dates and names, however, are not compatible (there are for example two Johns born in 1802 and 1808 - the first one didn't die before 1818; there are also two siblings born in May and November of the same year etc).

Now my question... There are clearly two James' Hendersons, both living in St James (or thereabouts), both married to a "Ann". Is there a way to tell them apart? How can I find out which child belongs to which James/Ann?
In other countries it is sufficient to look at the wife's surname, but the UK has this nasty habit of never mentioning it ever again after she's married :)

And related to that: is it more probable to think that James was actually born/baptised in St Martin's in 1813, and that the birth year (1811) on several censi is wrong? Or is it more plausible to think that however he was born in St Martin's, he was baptised in St James and that's why it is different on the census and baptism records?

Thanks for any help!