Any organisation that deals with vulnerable adults and children are required by law, to carryout an enhanced disclosure on any person coming into contact with the children/adult.
If the nursery wants to have you as a volunteer they can do this, and should carry out the enhanced CRB check - this is an expensive process and private nurseries may not want to do i for a volunteer. Why not look for council-run nurseries and summer playschemes where temprary workers are needed and the CRB check comes as standard?
You don't always need a CRB check for helping in a nursery. Only if you will be left alone with children or taking them to the loo. Parents often help out at playgroups on a rota basis and aren't CRB checked. Try looking on www.do-it.org.uk to see if there are any suitable volunteer places near you or else contact your local volunteer centre who wil be able to offer you more advice & guidance.
Actually thinking about it you would need a CRB check as you would be having regular contact with the children, rather than a helping out parent which would be a one-off. sorry for the confusion - it's too late at night for me!