I had no trouble getting the arrangements I asked for both times, I went from full time to 3 days per week with DS1, and then after coming back from leave with DS2, I was asked to do a 7 day fortnight, which I had no trouble with.

I've even been able to change all my start and finish times around to coincide with being able to share the school pick up and drop offs with DH, they've been really supportive and never knocked back anything I've asked for.