For use with the 'Advanced Special Offers' extension.
The requirements are:
Quantity of 6 items to be purchased for the price of 5, from a product setup which is configurable with associated products. The purchase may be made up of any quantity of the associated items.
Use the "Each N-th" action
In the SKU condition at the bottom you should define SKUs of configurable product itself and all its options that are applicable to promotion.
In this scenario, 'Fixed Discount Price' discount, would also achieve the same results. £x off per quantity of 6.