the Inland Revenue has produced the following pdfs as support for employees considering the use of childcare vouchers:

Parents registration form for you to print (93k)
Full guidance on salary sacrifice (PDF 32K)
Tax credits calculator www.taxcredits.inlandrevenue.gov.uk/HomeIR.aspx
Check if your childcare provider is approved at www.ofsted.gov.uk/childcare

other useful links…

You may require adobe acrobat to view some files - you can download it here