Been a grey area for years this, but its only grey for those that have self employed drivers working solely for them and try and justify it. For me its always been black and white.
If you take on a driver who works solely for you and nobody else, then that driver is not self employed.
If you expect him to wear you uniform, have your logo emblazend on the side of his van, and expect him to work for you and you alone, he is not self employed.
If you supply the vehicle and pay him a day rate, he is not self employed.
If you pay him a retainer to sit around and wait, to cover a particular customer or area, then he clearly cannot do anything other than take work from you. Thats what you're paying him a retainer for. So again, no longer self employed.
If you get the 'ump with one of your sub-contractors cos he's not available when you need him, because he's had the audacity to get a job from elsewhere, then keep him busy and he'll have no need to look elsewhere. But if you expect him to be available just for you, he is no longer self employed.
You can't be self employed if you get work from one source. Easy