For about ten years I had been unable to find an ancestor in the 1851 census, until a reply from a less blinkered RootsChatter showed that he had been hiding in plain sight.
Ralph and Elizabeth married in early 1849 in Newcastle on Tyne, witnessed by Thomas and Sarah. That finding, now confirmed, was in some doubt because her name was recorded as Andrews instead of Anderson. We knew that Ralph was born in 1816 and Elizabeth (his second wife) in 1825. In 1851 she was with her first child at a distant aunt's, with no sign of Ralph. Eventually I found him with Thomas and Sarah, still in Newcastle; Ralph's age was given as 28 (actually 35) and the others 38 (close, but not exact). Sarah, who had married Thomas in 1836, was Ralph's sister, 6 years older - so she should have known his age
.
The moral of the tale is to allow for a wide margin of inaccuracy when searching. I had probably asked for birthdates within 5 years of 1816, which wasn't enough.