wanda - I get the vouchers that cacaboo mentioned. And yes it is only if your employer signs up to it (in government parlance it's "employer supported childcare". But as they actually save employers NI on the amount too, it's a saving to them, so worth lobbying for it - if you're public sector they'll likely be amenable to it. There's some info for employers on the Daycare Trust website if I remember rightly. See \link{Talk?topicid=6&threadid=41524#957914\here.
The exact savings will depend upon your tax/NI status. For higher rate taxpayers who aren't contracted out of SERPS - assuming you have the maximum £218 in vouchers - it could be up to 41% of £218/month (i.e. £89.38). If you're in the normal tax+NI band it would be 33% (i.e. £71.94). Lower tax band and/or earning less than the NI limit per month, or in the gap area between the NI ceiling and the 40% band, and the savings are less obviously.
However these are per parent, not per child, so get your partner to lobby for them too, and you can double up on the savings.
One thing you might need to check is how getting vouchers might affect your entitlement to tax credits. Childcare vouchers supplied by your employer are NOT considered as income for the purposes of calculating tax credits, but neither is a voucher spent considered to be expenditure on childcare.
So if you would normally qualify for the childcare element of child tax credit, then you'd need to do some calculations to see the effect that having the vouchers would have - try plugging the with and without figures into the online calculator on the IR site and see what it comes out with.
If you qualify for CTC, but your joint income is high enough for you not to get not the childcare element, then the vouchers could be doubly worth it, since as well as the tax saving they reduce the income CTC is calculated on, so possibly increasing your CTC (although the band to get only the minimum is quite wide so this won't work for everyone).