I would agree with JMO (And I too have a lot of experience in that game).
By going through a contractor (you think) that you can take the odd day off or go on holidays, the contractor might have a very different view of things.
If'n you get your own run then surely you have a mate/s who you can pay the full rate + an extra few bob to cover your odd day off.
With that option you would be way better off than paying a contractor fer doing next to nothing.
I used to cover runs fer mates before, but all they offered was the standard price less the contractor/base fees and no Extras fer doing them a favour. I don't do that anymore.
If'n ya want a mate/colleague to cover a run, then offer them the full fare + a Tenner on top and then they will cover it fer you and pay them ASAP just to keep them onboard.