There is this Cottle Family in Radstock, Somerset in 1851
As you have Anc. iolaus I'll type up a brief outline.
John Cottle, 25, Head, born Radstock
Clena Cottle, 20
Adelaide Julia Cottle, 1
Charles Cottle, 18, Brother
Sarah Brice, 12, Cousin
MARY Cottle, 58, Mother, Widow, born High Littleton, Somerset (1793)
JOHN was baptised 04 Sep 1825, St Nicholas, Radstock to JOHN/MARY
Fathers Occp Labourer
+
GEORGE 11 Oct 1829
CHARLES 10 Feb 1833
1841 we have;
John Cottle, 55
Mary Cottle, 45
John Cottle, 15
George Cottle, 11
Charles Cottle, 5
From Genuki;
"HIGH LITTLETON, a parish in the hundred of Chewton, county Somerset, 8 miles S.W. of Bath, its post town, and 10 S.E. of Bristol***. The parish includes the hamlet of Hallatrow. A large proportion of the people are employed in the collieries.
Aaah found a site with Records for High Littleton;
http://www.highlittletonhistory.org.uk/registers.htmlQuite a few Dando entries there including;
1792 Jun 17 Mary d John DANDO & Isset
Just a possible scenario but hard to confirm if it is your Mary, Mother of Robert.
EDIT Baptism entry is also on FreeREG and John/Issets Marriage 1789
Scrap that as Mary 1793 must have died as they Bapt another Mary 1798
May help someone else in the future but looks like Mary Cottle is not yours