There are several ways to invest this voucher. If it is not claimed in the childs first year of life as Neil says the government set up the Stakeholder acount for the child and send the details to the parents, they can then change to another account if they are not satisified with the one opened for them. I think they get another amount when they are a little older and they are looking into a 3rd amount at 7 years of age.
http://www.childtrustfund.gov.uk/Homepage/fs/en