Schweppes UK no longer exists so I would think the product was canned in Australia. Either the seaming machine, probably 12 heads, had one or two faulty heads or some of the lids lacked the thin layer of artificial rubber compound under the seam edge. Schweppes QC usually find the former when they check cans numbered according to the machine heads and if faulty seams are found the batch is quarantined. The latter is more difficult because packs of lids containing faults are down to the can/lid producer who have their own QC program. If one wants to return this complaint to the shop, take pictures of the any contaminated storage area before removal. Don't touch with the bare hands as the fungal/yeast growth can be easily be transmitted to other food items. Contacting the address printed on the cans may be better than taking back to the shop as you will get instructions on how to return items (lids and cans will different ID marks and of course different to the canning date) and you are more likely to receive a larger replacement voucher than a shop will give.