Neither do I, Colin.
I am considering this...
St Mary Gate, All Saints, Derby, Derbyshire
David Welch, M, 30, Attorney
Ann Welch, F, 30
William Welch, M, 5
David Welch, M, 3
Ann Welch, F, 1
George Welch, M, 6 months
Mary Dennis, F, 15, F.S (female servant?)
Her death certificate from 1883 gave her age as 60, making her year of birth as 1823. If she was about 18 years old in 1841, would her age have been rounded down to 15 for the 1841 census?