Thanks for that comment. If I knew that then, I'd have offered that too.
I just thought that I'd offer the minimum and negotiate if necessary. I didn't think that the nanny would accept it silently and resentfully, and then explode. Anyway that's going off the topic...
Back to the title topic, I find calculation of bank hols is quite complex for part time working. For instance, in 2008 there are 4 BH Mondays, 2 Fridays, 1 Tues and 1 Thurs. So if you just do "20 days plus paid leave on any BH that you would have worked" then it makes a huge difference whether you work Mon+Thurs+Fri= 7 BHs off, or Tues+Wed+Thurs= 1 BHs off.
I didn't think it was fair that one should be penalised for the particular days of the week they worked, so I tried to smooth out this discrepancy by stating that the holiday entitlement is inclusive of BHs that they would need to take, such that the total number of days off would be the same regardless of the days you worked. But it seems that the nanny wasn't interested in any of this.
I think they would have been happier with "20 days + any BH that they would have worked under normal contract hours", even though it would have meant fewer holidays. I've learnt my lesson.