Dale
I've been looking more at the Mercer question. There's a description of the coats of arms used at the memorials in the church at Tenby, Pembrokeshire. That should be helpful for identifying which families were in Perrott ap Rhys' lineage.
The page is http://books.google.com/books?vq=Mercer&dq=Laws+and+Edwards,+Ch...
And it reads:
The Scotsborough monument is very rich in heraldry. On the upper part is a shield, on which are marshalled the coats of Ap Rees and Mercer, baron and femme, the former with its eight quarterings; below, on the dexter side, Ap Rees, and on the sinister Mercer. The quarterings of Ap Rees are as follow:
1. Ap Rees.—Argent, a chevron between three ravens sable, within
a border componey azure and or. He inherited the ravens from his grandfather, David, who was a natural son of Sir Rhys ap Thomas.
2. Mart1n or Cemmaes.—Or, two bars gules. Thomas's great
grandmother was Alsonde, daughter of Arnold Martin, of Richardston.
3. Marles Of Marloes.—Or, six martlets gules, three in chief,
three in base. Arnold Martin's mother was daughter of Richard Marles.
4. Batman Ok Honkyborough.—Sable, a chevron between three
escallop shells or. Thomas's grandmother was Elizabeth, daughter of Thomas Batman.
5. Perrot Of Scotsborough.—Gules, three pears pendant or,
on a chief argent, a lion issuant sable. Catherine Perrot, Thomas's mother, was representative of the Scotsborough branch of the Perrot family.
6. Verney (?).—Quarterly, azure and gules on a cross or five ermine
spots (?). Catherine Perrot was fifth in descent from Isabel Verney. See monument No. III.
7. Leve1.ance (?).—Gules, on a chief indented sable three martlets
or. Isabel Verney's mother was Elinor, daughter of William Levelance.
8. Rocn.—Gules, three fishes naiant in pale or. Elinor's mother
was Lucy, daughter of Thomas le Roch.