Since relationships aren't given on the 1841 census you can't be sure Elizabeth was William's mother - she could for instance have been an aunt. Neither unfortunately do you have status, so you don't know whether Elizabeth was married, widowed or single. You could hopefully ascertain her status from her death certificate as well as her age.
The National Burial Index gives this information about Elizabeth
12th March 1844 Elizabeth DIMMOCK 92 Herts King's Walden, St Mary
If this age instead of the census is her correct age this would give Elizabeth a birth year of circa 1752. She would have been 45 in 1797.
Other Dimmock burials in King's Walden 1800-1851 on the National Burial Index were
23rd March 1825 Mary DIMOCK 10m Herts King's Walden, St Mary
17th May 1829 Louisa DIMMOCK 30 Herts King's Walden, St Mary
14th August 1829 Henry DIMMOCK 15w Herts King's Walden, St Mary
22nd March 1836 George DYMOCK inf Herts King's Walden, St Mary
IGI marriage
WILLIAM DIMMOCK
Spouse: LOUISA SHARP
Marriage: 02 DEC 1820 St Mary, Hitchin, Hertfordshire
IGI Baptisms
FRANCES DIMMOCK
Christening: 07 OCT 1821 Kimpton, Hertfordshire
Father: WILLIAM DIMMOCK
Mother: LOUISA
MARY DIMOCK
Christening: 27 JUN 1824 Kings Walden, Hertfordshire
Father: WILLIAM DIMOCK
Mother: LOUISA
ANNE DIMOCK
Christening: 30 JUL 1826 Kings Walden, Hertfordshire
Father: WILLIAM DIMOCK
Mother: LOUISA
Regards
Valda