While I was googling around trying to find anything substantial about Sir John Mortimer (and failing miserably) I did find a very interesting article about Owain Glyn Dwr and his links with the Scudamore or Skidmore family. Actually it goes into all sorts of details, and gives a full list of Owain's children. Well worth a look.
It's on a PDF which can be located via the Scudamore/Skidmore Family History Site. The Occasional Paper you want is:
13. A REVISIONIST’S LOOK AT THE SKYDMORE-GLYN DWR ALLIANCE.
Recommended for anyone interested in Owain.