Having recently returned from holidays, I was pleased to find Mabel's death certificate waiting for me in the post. As I suspected, she died in Wales. I had heard that she had been living in Bristol with husband Edward (Jack). It turns out that Mabel died on 15 October 1970 at the Energlyn Hospital in Caerphilly.
This is where Energlyn is located in Caerphilly:
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Caerphilly is about 17km north of Cardiff.
Mabel was age 88 at her death. Her occupation was given as "widow of Edward John Newton" and "personal secretary". It appears that her last address was "Brodawel" which was a care home in Energlyn, Caerphilly. Her death was registered by Charlotte Agnes Nicholas, cousin, who lived at 140 Plassey Street, Penarth, the former home of their grandfather, William Huxtable Ball.
The cause of death was given as "acute bronchopneumonia" and cerebral arteriosclerosis.