The 1841 and 1851 census for Wheathampsted indicate that Thomas EPHGRAVE was born around 1786-7 in Hertfordshire (Wheathampsted specified in the 1851 census). The IGI has a member-submitted christening for a Thomas EPHGRAVE in Wheathampsted in 1787 (but I suppose this could just be based on a reading of the censuses above). RootsWeb WorldConnect Tree has an entry for Thomas EPHGRAVE christening in 1792 at St Mary Luton, Bedfordshire. The IGI also shows this christening, with Thomas son of John Ephgrave and Sarah. The same (possibly) parents are also shown as christening a Thomas EPHGRAVE in the same place in 1795, which might suggest the first Thomas died young.
The WorldConnect tree doesn't give sources, unfortunately, but does feature a lot of data about the children of Thomas (and his siblings), including the three children (Sarah, John and Frederick) mentioned in his wife Elizabeth's will proved in 1855. Having found all these data I was quite excited, but now they seem to pose more questions rather than offering answers! I guess I really do need to visit HALS again and the Bedfordshire archives in Luton to try to fit the pieces together. One thing seems clear though; Thomas EPHGRAVE and his wife Elizabeth KILBY had more children than the three mentioned in Elizabeth's will, but they all died in infancy, in some cases buried on the same day as younger siblings were christened. One child seems to have lived to her late teens, and I've ordered the death certificate to see if there really is a connection.