I see that I missed a 1 from the 1851 ref - should read HO107/2509/621/4-5
If the marriage is them, then the search would be for baptisms of
Thomas Roberts b.c. 1789
Ann Jones b.c. 1786
The censuses just say b. Denbighshire so a broad search in Llanarmon DC , Glynceiriog, Llansillin, Llangollen, Llanrhaeadr YM or other parishes in vicinity.
I'll have a quick search but have lots to do.
Gadget