Thanks Jen, I'm almost embarrassed to ask this as I consider myself a bit of a whiz at these things, but how do you search by the reference ? I can't see anywhere to put that .. I tried search the 71 census for his name in St Pancras but it just doesn't come up for me :s
The reference is RG 10 / 218/ 65 / 49
So go to the 1871 census search page on Ancestry.
Scroll down the page a bit and in 'piece number' enter 218
In 'folio number' enter 65
In 'page number' enter 49
You need not enter anything else.
On the first page of results you will find Benjamin Jones (but not transcribed as that

, which is why your search isn't turning him up

)