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I stopped at the Customer Services desk on the way out, and spoke to the chap manning it. He basically backed the assistant's theory up, but added that strictly speaking I wouldn't be allowed a flimsy bag for the fish anyway, as in theory, a sealed tray shouldn't leak.
He then went on to say that some folk were using these flimsy bags to carry all their shopping because they weren't prepared to pay for a proper carrier bag.
I must admit I came away feeling a little bit miffed, as I felt that in my circumstances I should've been given one, but when I got home and thought about it, I wondered if I'd been unreasonable. :o/
Chris, yes, the flimsy bags did have handles. I'm a bit tied up now but I'll read that link later, thanks, and thanks for other replies, peeps.