8 till 7 is OK hours if you are the boss and are prepared to put in the time and don't have a life outside of work. If you were an employee and were expected to 10 hour plus days with 30 minutes break, you would soon get fed up and jump ship.
I believe you have to have a balance. For example, why not make the hours 8 till 4 with an hour's lunch, paid if they meet targets as a bonus, and unpaid if they don't.
Your wage bill would be reduced to 304 quid a week with the lunch paid bonus, your workers wouldn't be knackered from working excess hours, and so would be fresher. And they could have the evenings off.
John