Hi Galwaymum,
I'm a civil servant also just back from maternity leave. I found the best route initially to be parental leave which can be taken as 1 day a week (or 2 , 3 whatever way agreed with your supervisor). It allows you to see how part time will suit you before you make a move to working part time offically. Like you I felt I would love to stay home full time with my son, 11 months now, but when I returned to work last month I've found myself enjoying it so its nice not to have to make my mind up about going on an official 4 day or 3 day week yet.
Regarding money I have found like the OP that I'm down about 16% for 20% less hours but there are factors like commuting costs and childcare costs which make it quite efficient for me at the moment to be on a 4 day week. Parental leave works out at about 70 days so I have another year and a half almost before I have to decide what I'm doing.
Best of luck and enjoy....