Rich
They are taking the Pee big time.
I have seen it all before, to find out who it is, send them on there own, if they can't average over £200 a day each, more on good work, then you have seriously got something wrong.
Its your business and take control, make them work 8 hours if they finish early give them more work, there are plenty out there who give there all for £8.00 per hour.
If it were my boys, they would be getting an ultimatum in the morning