Not all of them are. From http://www.sitters.co.uk/questionsanswered.aspx#A9 :
Q: Are all Sitters babysitters police checked?
A: More than half of Sitters babysitters have experience working as a Registered Childminder and would have been cleared through the CRB (now DBS) as part of their...
Blimey - if that business idea rocked up on Dragons' Den I world have laughed it off that no one would use it.
Does the babysitter ever turn up after the kids are in bed, with the risk they wake up to find someone they've never met before?