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Viv, it was the other way round - the checkout girl said I could have a free bag for the fish but not if I was going to put the frozen mash in it too (although when I asked at Customer Services I was told that I didn't qualify for a free bag at all, as the fish was sealed and in theory shouldn't leak).
I'm not against the idea of paying for bags (although I always reused my free bags as bin liners) - it just seemed a bit picky to tell me I couldn't put the frozen mash in the bag along with the fish, unless I was willing to pay 5p and call it a carrier bag. I was clearly not one of those who wanted to carry all their shopping in several of those little bags to save buying a 5p carrier bag (which is rather cheeky and I can fully understand a shop putting a stop to that).
Both checkout girl and Customer Services stated, 'We can be prosecuted if we don't follow this through, we have pages and pages of legislation on this'.
Ah, well...wouldn't want 'em prosecuted for breaking the law...I'll start carrying plastic as well as cloth bags from now on. :o)