Hi Hazel,
No record of a baptism for Mary to link her with Adam and Elizabeth im afraid.
I searched OFHS Letcombe Regis PRs and this is what i found

Adam and Elizabeth did only have 2 children
Thomas born Aug 15, bap Aug 20 1758
Ann born Apr 1, bap Apr 7 1760
There is no marriage here for Thomas and Mary so must be Stanford in the Vale

There are 12 children for a Thomas and Mary but names spelt Froude, Froud, Frude and Frowd

Here is Ann's marriage, note spelling of Richard

Apr 16 1781 Richard Skillin, otp, to Ann Frowd (x)
Wits James Howard and Martha Smith
Also found i child for Richard and Ann, Mary Ann Skillin
Here is a burial, only one for name Skillin
May 3 1797 Ann Skillin
Does this all match with what you have Hazel?
Sue