All the information you can give helps.
John was baptised 28 December 1817 in Llanbadarn fawr Cardiganshire. In the 1851 Census he is at Tredegar Road Row, Rhymney Iron Works. He married Mary Owen from Ambleston Pembrokeshire on 2nd August 1853. The 1861 Census he is at Glyn Cottage, Cwmavon, Abersychan with family. In the 1871 Census at Hen Felin, Cwmavon, and the 1881 census at Hill Street, Ebbw Vale. He's listed on the 1887 Electoral Role at 3 West View Terrace, Ebbw Vale. Mary died at same address on 22nd September, 1889, shown to be a widow.
Thanks for looking
I think this may be your John's death registration (he seems to be the closest fit to your John):
Name: John Davies
Estimated birth year: abt 1816
Registration Year: 1887
Registration Quarter: Apr-May-Jun
Age at Death: 71
Registration district: Bedwellty
Inferred County: Monmouthshire
Volume: 11a
Page: 64
Ebbw Vale was part of Bedwellty registration district until 1894, as far as I can see.
I believe the General Register Office is putting some certificates on line for a reduced cost - not actual certificates, but show all the important information. It might be worth having a look.
Good luck
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Thanks for trying but this person lived in Charles Street, Tredegar and was a Collier. (I'd already purchased the certificate).