No COLLENS on 1851 Cambs census index. Found these at Soham.
Surname Forenames Age Parish or Town Film Folio
COLLINS William 24 Soham HO107/1763 149A
COLLINS Thomas 44 Soham HO107/1763 149A
COLLINS Sally 39 Soham HO107/1763 149A
COLLINS George 4 Soham HO107/1763 149B
COLLINS Lucy 2 Soham HO107/1763 149B
Found these on 1841 Cambs index.
Surname Forenames Age Parish or Town Film Folio
COLLEN Thomas 45 Soham HO107/73/18 6A
COLLEN Thomas 13 Soham HO107/73/18 6A
COLLEN Thomas 10 Soham HO107/73/18 6A
41 & 51 index online plus other useful Cambs links;
http://www.cfhs.org.uk/Search.html
Also Cambs BMDs online;
http://www2.cambridgeshire.gov.uk/db/ridmars.nsf/search?OpenForm
Regards
Paul