My DD's goes to a lovely CM 2 fixed days pw plus a Tuesday every other week to fit in with my working pattern. As the Tues is only fortnightly I came to agreement that I would pay the CM a retainer of £13 for the Tuesdays that my DD is not there. This was because it meant that she would not be able to find someone also wanting alternate Tues, and also so that I would have the flexibility of using that day when needed to work extra days at work when required or if I upped my hours on a permanent basis.
So far on the odd occasion I have needed to utilise the spare Tues, it is has been fine and I have paid the extra to make it up to a full day's rate. However, when I told her that I would need my DD to go to her tomorrow as I was needed at work last minute she said she had no room (even for a half day) as she had a couple of new starters that she had booked in for settling sessions. This has left me with a real problem at work. AIBU to feel this is unreasonable even though I only informed her the day before I needed the Tues?