Sorry to spoil the fun, but this offer is not as good as it first appears as it only equates to 7% in real terms. This is because they offer a 33% discount anyway as all their stock is buy 2 get the 3rd free. The voucher usually doesn't include this discount and is just off their single bottle price which is higher then the equivalent price on the buy 2 get 3rd free. They basically penalise you for buying a single bottle, so the voucher is not as good as it first seems.
Complicated but I hope that explains a bit!