Another way at looking at it is that a day rate is called a salary, that’s a set rate in other professions
Any hours you go over is given as time in lieu, ie holidays,
All well and good when you’re working in a job that the contract says 40 hours a week but that’s Never going to be in a drivers contract who is on a daily rate
It’s an outdated way of paying a wage just like the way trip money is now
If you work for a haulier that pays only a days rate you don’t have any come back, ie doing all the hours that suits him not you, and it usually ends up your working for below minimum wage
If it was me I would work out max hours a week and then compare it to what he’s paying and that should give you an idea of how much he can run your into the ground for peanuts