How will 45 hours per week be split over a month? Is it say 45 x 52 / 12 = 195 hours per month?
If you have every Friday off during term time, then that I think is around 180 hours a year that you need to fit somewhere else.
If you are not working on a Friday, then you are not working. It is up to the parents to arrange for a sick child to be collected from pre-school if it occurs that day.
Why are you not needed to work on Friday's during term time? You were doing 8-1, pre-school I presume is something like 9-12. So presumably one of the parents is at home that day so can take to pre-school and collect from pre-school.
I get 20 days holiday and the 8 bank holidays: 10 days of my choice, and 10 days of their choice.
How is that phased in your contract?
An employer can dictate when you take holiday.
As you work a different number of hours on different days, you can't really calculate holiday as number of days... it should in my view be done in hours. Currently if you took a Thursday off, that is 10 hours. Whereas if you took a Friday off that is 5 hours.
The family are having 11 days holiday in the summer, as it's 1 day over my holiday allowance they've asked me to make up the hours.
They are going to be away for 11 working days... so that's a little over 2 weeks. Their choice to go away for that period, they can ask you to take your holiday at the same time, so I think your argument is that it is going to mean that you only get 9 days of your choice left this holiday year.
What happens at Xmas? I expect they may choose some more days, or you will choose not to work over the xmas-new year period.
I think this is unfair, as it's not my fault I can't work on this day, it's their choice. Am I being unreasonable?
If your contract specifies that they only get 10 days of their choice, then any days over that I feel they should be paying and not expecting you to do additional unpaid work.
If the contract says something different though, they can reasonably request that you take that time off as part of your holiday entitlement.
Would you have taken no time off at all over Summer if the family had not been going on holiday? I expect you may have requested some time off, so is it that unreasonable for all those 11 days to be considered their choice?
I do not like this choosing 10 days each thing... it does not work in my view. Ideally an employee should be able to choose some of the days they take as holiday, splitting 50/50 I feel is a recipe for disaster as you are finding out - there are problems if one side feels they have not been given their half of the choice days.