Now indexed and searchable, the 1851 and 1861 census transcripts are the latest additions to ancestry.co.uk. However, images of the originals are not available,likely as copyright clearance could not be obtained.
Don't forget that there are often public access locations where you can access ancestry, including the Ottawa Public Library and Ottawa Family History Centre.
These databases augment the 1841 census of Scotland, also available through ancestry, and the 1881 census which is on CD at the BIFHSGO library and Family History Centre. Don't forget to check FreeCen which has census data transcriptions for several Scottish counties for 1841, 1851 and 1861 at a price that's hard to beat.