Hi,
I would think this is the one.
In the Appleton transcriptions (from the OFHS);
Anne Coster is recorded as being baptised 11 Jul 1779 the dau of John & Mary.
There are six other children bapt. to them and I can list easily if required.
Also; John COSTER, bach, married Mary SAYER, sp, 19-12-1775 Appleton.
In the Dorchester PRs there is this marriage entry;
20-6-1779 John COSTARD to Mary BOND by Lic. (followed by presumably the witnesses);
Vinct. Gosford, Sarah Thompson, Mary Borton.
Interestingly I could not find any children born to them in Dorchester
Hope these help.
Alan