I think you need to do some more work on Thomas from Gamlingay before accepting that he's the one who married Elizabeth Baringer. Did he die as a child? Are there any other Thomas Harper marriages which might be him? The witnesses at the Abbotsley marriage don't add much - Jonathan Lincoln and William Cade, Clark (William acted as witness for most of the marriages at this time)
A website which you may already know of is
http://www.cfhs.org.uk/Search.html which has baptism and burial indexes for Cambs 1801-37. The burial index shows a John Harper age 60 buried at Gamlingay in 1818, but no sign of Lucy.
I suspect that Lucy may have died at some time between 1798-1800 as there's a marriage of a John Harper to Susan Collins in Gamlingay in 1800. You would need to check it out to see if John was named as a widower, and if there's a burial of Lucy and of any other Harpers 1787 onwards
In 1841 a Susan Harper age 63 not born in Cambs was living in Gamlingay in the household of William 50 and Martha 39 Jones. William Jones married Martha Harper in Gamlingay in 1822, and Martha was the daughter of John and Susan, baptised in 1802.
David
