When you get David's marriage certificate you will/should get:
- His father's name. Will be helpful to know if Meurig/Meyrick was actually his middle name and there was another name that he might also be recorded as. (The William M. looks very possible). - His father's occupation. Might help identify him on censuses. (Though people could change occupation of course). - Whether father was deceased at time of marriage. But note that sometimes this isn't recorded.
Good luck! I think you're goimng to need it! Without at least some of this information it's going to be virtually impossible to identify David on the censuses - there are SO MANY David Thomas's in the Merthyr Tydfil enumeration district!