If I were youy I would contact the Trading standards Office if they don't stump up the bag or at lrast offer you something of compartable value. Firms have gone bust over having to honour offers like this, but they obliged to do so.
A few excerpts from www.gov.uk/.../system/.../10-1312-pricing-practices-guidance-for-traders.pdf -
"Free offers and similar promotions"
"1.10.1 You should note that one of the practices which is banned by the CPRs in all circumstances is describing a product as “free”, “without charge” or similar, when the consumer has to pay anything other than the unavoidable cost of responding to the commercial practice and collecting or paying for the delivery of the item."
"1.10.2 You should make clear to consumers, at the time of the offer for sale, exactly what they will have to do to get the "free” or “reduced price” offer."
If you have done what was asked of you, they should fulfil their part of the contract, or it is null and void and you should get your money back - not be asked to spend more
"1.10.4 If there are any conditions that are part of the offer, you should give at least the main points of those conditions with the price indication and make clear to consumers, before they are committed to buy, where they can get full details of the conditions."
^"1.10.5 You should not claim that an offer is free if:
(a) you have imposed additional charges that you would not normally make;"^
Such as asking them to place two orders instead of one!
