It is certainly something you can do. My current employer found me without using an agency, they posted an advert at NannyJob.co.uk and at NannySelect.co.uk I think they interviewed 5 candidates, don't know how many enquiries they had, but it was at least 5!
Your location may make a difference - as publications in which you can advertise, do not cover all area. Simply Childcare for example is mostly for London. Nanny websites tend to be national, and general jobs boards tend to be regional, or city based such as Gumtree.com
You can also advertise locally in newsagents/postoffice, at school perhaps, local NCT or other parenting type newsletter.
CIS do not CURRENTLY list nannies. Laws change in April 2008, which will mean that from April 2008 they will start listing all childcare providers. Currently it seems to be up to each CIS who they list, as my neighbouring area (Berkshire W&M) CIS do list me, but Surrey (where I actually live) refuse to do so, even after I complained.
For anyone interested in the new law, see Childcare Act 2006 .
£6000 for a nannies salary seems a bit low, if your 10% figure was right. Will depend on days/hours of course.