Can't see him in the wills section of the IDD although there is a Jas Seaman of Heigham in 1837.
Thanks for the tip about the military record for drummer Wm Seaman born 1766 - it's a militia record. It seems to say that he joined 11 Jan 1804 at the age of seven! Which would make him born 1796, too young to be my William sr.
then it says he served for 22 years after the age of 16, up to 1835. Which again would mean he was born around 1796/7. And on the second image it says that he was 18 on 11 Jan 1815.
I wonder whether findmypast will refund my credits!