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Tell them thanks, you'll take the free half of the pair they are offering, to match up with the half pair you have at home.
Then wait and see whether the penny drops...
ha! awesome. I may just have to do that!
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I hope you have a receipt of proof incase they go all dodgy and pretend they never had it...
Nup. Im hopeless at keeping receipts..never do

So, no legal ground to stand on i guess.
just wondering if there is anything else I can do bar getting in a huff and flouncing telling them Ill never buy from that shop again?