We pay a mixture of flat rate and bonuses. Employees might not get pay equivalent to the minimum wage every week, but they probably would over the month. Is this acceptable?

You must ensure that you are paying the minimum wage during each 'pay reference period'. This is normally the period over which you pay people - for example, weekly or monthly. It cannot be longer than a calendar month. However, you can base your calculation not only on the amount paid for that period, but also on the amount earned during it - for example, by way of bonus or commission - provided that the extra earned is paid in the next pay reference period. Obviously you cannot then count that extra towards the minimum wage in the second pay reference period - that would be double counting.