You can calculate yourself roughly whether you will get any of your childcare paid through tax credits here:
http://www.taxcredits.inlandrevenue.gov.uk/Qua lify/DIQHousehold.aspx
That way, you can model a few scenarios to see what works out best.
Personally, i think that nurserys are taking advantage of the fact that parents do get some help by putting up their charges.
When my two were little, the cost of childcare was higher than my husband earned as a nurse, so he gave up work to look after them full time and we never claimed any benefits. Tax credits weren't around then though.