The naming system you refer to is “Welsh Patronymic” where the forename of the father becomes the surname of the son, what is missing is ‘ap’ ('son of' or just 'of') therefore David Pritchard (a corruption of Richard) is in fact David Ap Richard Ap Dafydd and David’s father is in reality Richard Ap Dafydd Ap Llwyd.
For the lineage of DLG I would refer to ;http://thepeerage.com/p19916.htm
about half way down the page.
Patronics where popular in strong welsh nationalist areas of the country pre 19th century, however one place that it is never found is here in “Little England beyond Wales” or sometimes referred to as S.W. Pembrokeshire.
ORIEL a welsh window on a surname