Okay finally found Robert in 1841 census.
His age is pretty consistent throughout 4 in 1841, 14 in 1851 and 23 in 1861, so 1837/8
His brother Thomas was actually born 1844 registered Downham, although not baptized until 2 years later, and Stephen was registered 1848 and not baptized until 1852, both have no mothers maiden name listed so illegitimate as with baptism records.
Ref new GRO index.