Try some of the free papers that the various publishers have on their pages - they are mentioned on the resources pages here http://www.elevenplusexams.co.uk/revision_aid_download.php
under the free practice papers on the LHS.
Also schools like HABS http://www.habsboys.org.uk/info/examinations/11plus/11plus.php
have some great papers worth looking at without a time limit, maybe just a few questions at a time. Also I think the national maths challenge papers at http://www.m-a.org.uk/jsp/index.jsp?lnk=251
are worth a try particularly the November ones (the Feb ones probably go a bit further than 11+ needs)
We didnt feel that past SATs papers were of any use at all but a good strong knowledge of times tables & logic puzzles really helps.
See for example numbers 74 & 75 for logic type of questions (these are the sheets given out for extension work at school) http://publications.teachernet.gov.uk/eOrderingDownload/DfEE%200083%20200MIG1713.pdf