If you have family in Manchester then you will know all about the flood damaged 1851 census and how it was unreadable for years. The Manchester & Lancashire Family History Society (MLFHS) some time ago started a project to transcribe what they could of the census.
It was a long project and during the time it took to complete technology has advanced and recently Ancestry.co.uk have produced the best possible images and indexed them. However it is said there is some difference between the 2 indexes, so it is worthwhile checking them both. The MLFHS index is available for free and the Ancestry one is on their subscription site.