Do you think Mary died in Froncysyllte or Pontfadog?
Unfortunately I don't know.
I've lost the family in 1861 there is a possible Robert of a near age but the place of birth I seem to remember (I haven't got the detail and I'm not subscribed to Ancestry at the mo) was "North Wales".
So they are effectively missing for 20 years (1851 to 1871) Crogen then Fron.
They have ceased having children at that point.  The unmarried children of Robert and Mary are elusive in 1861 as well.  Robert is actually listed as married but I think this is wrong the error with his daughters relationship hints to this.
1871 (RG10/5682 p 14)
Cysyllte
Robert Hughes Head Mar 75  Shoemaker Salop St Martins
Jane Hughes 
wife Daur unmar 36 Housekeeper Denbigh Llangollen
Thomas Hughes Son unmar 28 Labourer Denbigh Llangollen
John Hughes grandson 12 Scholar Denbigh Llangollen
Thomas Hughes grandson 8 Scholar Denbigh Llangollen
Jonathan Parry brother in law 68 Labourer Denbigh Llangollen
Previous sighting in 1851...
27 Gelley Ucha   
Robert   Hughes   Mar   56   Head   Salop   Shoemaker
Mary   Hughes   Mar   44   Wife   Denbighshire   
Jonathan   Hughes   Unmar   26   Son   Salop   Stonemason
Richard   Hughes   -   13   Son   Denbighshire   
John   Hughes   -   11   Son   Denbighshire   
Thomas   Hughes   -   8   Son   Denbighshire   
Thomas   Parry   Unmar   66   Lodger   Denbighshire   
P 
