In the days following Daniel Diggle's execution, several regional newspapers reported that his mother has died two days later, on Friday 4th April 1817 of 'excessive grief'.
I have as yet been unable to pin down the source of this story, but the precise date derives from the Gentleman's magazine, an aggregator of news stories, which can be found online here.
In fact, Diggle's mother was called Sarah Diggle (nee Rhodes) and had married William Johnson Diggle in 1793. Genealogical records show that she didn't die in 1817, but was buried at St Leodegarius at Basford on 24th June 1827, aged 65. William died in 1841.
I am indebted to historian and genealogist Lesley Abernethy for researching Daniel Diggles's family.