Bad traders

You didn't look at the photos I published? You also didn't inspect the components before confirming that you were happy with your purchase? You bought second-hand components without considering that their condition wouldn't be as new? IT'S A CORRODED BOLT FFS.

Admittedly I didn't fully inspect the items, my mistake. You seemed like a nice guy so I trusted everything was in good condition. I even chose to overlook the massive smoke damage all the parts had because I was happy overall. Regardless of whether you think the corrosion a trivial issue, it spoils what is otherwise a groupset that looks in great condition. Just because its not important to you, don't presume that it isn't for another. For your information, the cheapest replacement anchor and bolt I could find is £9 and it's not even in stock. I found one that is and its £22 'FFS' (to quote you). Not cheap.

No, I'm excusing myself because, as I've already told you, I didn't know the adjuster was rounded-off because I didn't use it: I have readily accepted your word that it was in that condition when you received it. As such I would have refunded you £20.

Regardless, you should have checked everything was in working order before you agreed to the sale. If it was me, I wouldn't feel comfortable selling something for hundreds of pounds without first giving everything a proper check over.

I only fitted the groupset recently so, naturally, I hadn't noticed the issue up until now.

I didn't want your £20, all I wanted was for you to acknowledge that the part was unsuitable.