I work a supposedly 40 hour week, with commute time this becomes 45
So you need someone who is able to commit to doing 45 hours per week.
That person may be willing to do more hours on occasion, in exchange for less hours another week. However you cannot insist that an employee does additional hours. It is not fair and can be a fast way to lose a nanny if you regularly arrive home beyond the pre-agreed time.
Do full time nannies tend to like more /fewer hours? - I mean does the increase in pay more than compensate for the long hours?
Salary is dependent on working hours to an extent but it is not the only factor. Experience, location and what parents can afford can affect what parents will offer. Nannies will need to earn a certain amount to pay their bills. Live-out nannies will have all the usual expenditure as anyone else living in your area, plus travel to/from work.
What is full-time? When I worked for Government 36 hours over 5 days was considered to be full-time. In nannying, a full time nanny could be doing 60 hours over 5 days. Part-time nannies can be doing 40 hours or more. So probably better to ask if a nanny who wants to work 5 days a week, wants to do 12 hour days, or prefer 8 hour days. The answer to that will vary depending on the individual nanny, some of us like getting up late and doing a few hours work before going home, others get up at crack of dawn and hope to get home before midnight.
These days I am more likely to go for a job where there are more hours per day but less days per week as that means I get full days off. So much nicer to work 4 days rather than 5.
Is 'overtime rates' expected for time above normal working hours?
Generally I would say no, overtime should be rare. It does not occur very often so could well be at the same hourly rate. Though there may be exceptions to that, such as if it is on a non-work day, rather than just a couple of hours added onto a usual working day.
I personally prefer to have time off in lieu. So I do a few hours extra there, then another time and after a while it builds up to a day off. It may not match exactly on hours, sometimes if may be less/more than the working hours for the day that is had off but overall if everyone is happy it is fine.
Problems I feel occur when people count hours. Parents and nannies agree an hourly rate and then they both count those hours, sometimes down to the nearest minute. I feel it works better with some flexibility as long as neither side is seen as taking advantage. If you get home early, let nanny leave early. If you get home late, nanny is likely to be much more understanding that you were a few minutes late home.
Would a nanny agree to an annual salary for doing a maximum of a certain number of hours a year or a month? Maybe but once the maximum has been reached, they may not then do anything more than that. What if the amount was not reached... would they get the same pay as the max hours amount? Would you calculate holiday as being 12.07% of the total yearly hours? What would the number of hours be for a day off? It may start to get messy. So I would aim to fix the hours per week as best you can.
45 hours this week, 55 hours next week, 35 hours another week... I can't see you finding a nanny who would agree to that, or hours changing weekly/monthly. You might, anything is possible these days, but I feel it would make recruitment a lot harder. People tend to need a regular income not one which varies, so I feel you need to try to make the job appealing to as many candidates as you can by fixing the hours as best you can in the hope they may do some occasional swaps on your quiet/busy weeks.