• UPDATE: Went into the shop last night and explained the problem. Of course they couldn't do anything for me at the time (no mechanic - go figure) , but they've ordered a 3/32" cog which will arrive in a couple of days. So, on Wednesday they'll remove the incompatible cog and refund me for it, then fit the new one.

    I get the feeling they're not going to refund me for labour, or give me Wednesday's labour for free, but I'll argue that point when the time comes and see how far I get.

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