This can be quite complicated, especially if you have childcare costs (with a registered or approved childminder and therefore eligible for help through tax credits) which would change if you reduced your hours. There are not brackets as such - more of a sliding scale which takes all relevant elements into account - so it's not possible to answer your question in the way you envisage.
Where did you get the �40 p mth figure from? Was it based on your & your husband's income once you return to to work, or was it based on your joint incomes for the 06-07 tax year? If the latter then the figure may be too low if your joint incomes for this tax year will be less than for 06-07.
I doubt whether you will overall be better off if you work part time rather than full time, but it is not possible to give any more information unless you can post both the joint gross income for 07-08 assuming you work full time, and assuming you work part time. The average weekly childcare costs in both situations are also needed.