I think their a waste of money myself, even if the crb check is clear they just might not have been caught.. or they are now a reformed character :

A CRB check is vital if you are going to put domestic cleaners into other peoples homes. I would check your Insurance as well to see if they specify a CRB Check.
I think its just as or even more important to check background if they work in commercial cleaning, but be honest the time they can take, the cost, is it worthwhile..
Insurance, no not in my experience, mine requests that I get the employee 2 complete an application form, I also ask to see proof of address and identity

only because i'm nosey.
Anyway if you really need 2 know if their bent, dont waste your dosh.. you can get them done free if you know the right person