sian,west wales: yes Ll & Rh are exceptions to many of the mutation rules, but not the soft mutation.
Soft mutation affects C, P, T, G, B, D, Ll, M, & Rh
Nasal affects C, P, T, G, B, D,
Aspirate affect C, P, T.
It is the lack of the other two mutations which makes Ll & Rh appear to be exceptions.
I used the term "Welshified" because I don't know of any original Welsh place name of Rhymney, (as in the valleys,) or Rumney as a part of Cardiff. If they are invented place names then providing 'Welsh' spellings is the equivalent to providing English spellings of genuine Welsh placenames.

Nigel