You could try a private advert on any of the childcare sites www.nannyjob.co.uk, www.netmums.co.uk, www.childcare.co.uk, www.gumtree.com and also search for those looking for work on those sites.
Alternatively you could use an agency tinies agency cover cambridge and seem to have jobs popping up there alot or I recommend using www.kidsmatter.uk.com
good luck in your search.