A while back I wrote about CRB checks and suggested they are not quite as much use as you might be led to believe.
They have since been replaced by DBS (Disclosure and Barring Service) and from what I can see, at first look it doesn't look much different. However, I see that as of this month, for an annual fee (currently £13) you can register online for a DBS certificate (equivalent to being CRB checked) which is then available for anyone to check for no charge. The Update Service seems to let you autorenew that each year.
I still question the value of this in terms of the year's duration but it might put some potential customers' minds at rest.