Well I can't see much incentive to be self employed on that rate, once you've allowed holiday and taxes it would average out lower than the minimum wage but I guess that's why your doing it, legally it's a grey area as to whether your in the right or wrong regards self employment, however ethically you may be viewed in a different light
As a sub contractor/self employed you would maybe be better working out an hourly turnover you know can be done ( say £40 per hour ) then either give each guy £320 of work (8 hours) then they get 33% (£106)
That would be their cut regardless of time they actually take 5 hours or 11 hours
Darran