I'm sorry, I don't know how you get Australian certs - I was sent those that I have by someone who lives in Queensland. They have a lot of info on, as you can tell.
Re the 1871 census - all I can see are these:
John Dimmock and his wife Elizabeth age 82 and 79, living in Appleby, Leicestershire
a 29 year old John Dimmock and his wife Sarah
and a 3 month old John Dimmock.
I'd be surprised if the first was your John, if only because IME it would be unusual to have lived in the town (they married in Leic St Mary didn't they?) then move out to a relatively distnat village. The population usually moved in the other direction!
In 1841 I can only find one possible in Leicester -
John Demmocks age 9, born in Leicestershire, living in St Mary's in a household full of adult men, none of whom bears the same surname.
Class: HO107; Piece 605; Book: 8
(I hope I've put that reference correctly!)
This 9 year old John was a needle maker and the absence of anyone with a similar surname would make me wonder if he was an orphan.
