Hi Gwyn
The source is a family tree of 1934 vintage connected to my family which states that Thomas Jones (1665) of Llanfair Caereinion married 1690 Anne Wynne of Usena Abbey Melay in 1690, and she is now represented by T H Williams Esq Lord Newborough Aberglasyn Hall. Other people on this site have been very helpful in locating Melay (Melai). Maenan Abbey seems the obvious choice but Usena, if that is the name, still remains a mystery, maybe as you say it was a bad transcription.
Thanks
David