I don't wish to sound disparaging, but have assumptions been made here?
There is no baptism entry for a John Spilman Clarke who marries in this name in 1787 - there is a baptism for a plain John Clarke 13/4/1766 Lowestoft, son of Henry and Elizabeth and this seems to be the link to a Henry and wife Elizabeth.
However, unless you've found otherwise, there is no marriage anywhere in the country between a Henry Clarke and an Elizabeth Spilman??
It seems to me that because John Spilman Clarke married in this name it has been assumed (as Spilman carried on within the family) that his mother must have been named Spilman, and assuming he was the John Clarke bp'd 1766 Lowestoft to Henry and Elizabeth that Henry's wife Elizabeth was formerly Spilman.
There seem to be no actual facts to back this up - all that is definitely known is that a John Spilman Clarke married 1787 but there is no baptism in this name nor is there a marriage between a Clarke and Spilman with any Christian names as far as I can see.
Annette