Hi Rich,
In my Baptisms records I can find William, son of William (occ Scissorsmith) & Martha, baptised on the 26th September 1800,
and then John born 13th September 1802, son of William (occ Razorsmith) & Martha, baptised on the 17th October 1802.
Are these the 2 eldest children of your above William.?
And do you think it's possible that the London marriage is correct, they could have married out of Sheffield because it was already assumed that they were married when their first child was born