Thanks Teresa, a few more missing pieces of the jigsaw!
I trawled the parish records and found what I think are the christening records of "my" Thomas and Mary-Ann:
28/6/1772 Thomas, son of William Avern and Mary his wife
28/9/1793 Mary-Ann, daughter of Thomas Avern and Mary his wife
I think this makes them cousins (or possibly second cousins) the dates and large age difference tally with the later census records.
Thanks for the info that William Gardner married an Avern.
This would explain why Thomas sold him my house, built on the corner of the Malt House land, in 1817; Gardner had married his sister-in-law.