Cycle Republic

Hang on a minute here. If the shop are saying the pedals look like they've been disassembled and THEN washed their hands of the matter, it sounds like shenanigans on their behalf to me ie someone a bit overly ambitious at the shop had a play with them and realised they were in over their head. I'd keep pursuing this if I were you, definitely.

I've found Twitter good at times where the 'high street' location have been unreasonable.

You have fallen into a common trap here when you allowed the shop to deflect responsibility onto the manufacturer.  In law, your only contract is with the shop, the manufacturer has no duty to you and it is not your job to chase them.

I would return to the retailer and tell them the goods they sold you were not of satisfactory quality and that you expect a refund or replacement.  They owe you a reasonable response and having tried, and failed, to get help from Garmin they must now sort out the problem.

If all else fails threaten to report them to your local council's Trading Standards department, who would be delighted to hear from you.  

Your last throw of the dice is to threaten an action in the Small Claims Court.  This will cost you about £25 (if you fill in the form online, otherwise £35) and a judge will listen to your story and decide if the shop has been reasonable.  If not you will win a refund plus your court costs.  

If none of that works, as mentioned above, try your credit card for a refund and some debit cards also follow the same rules on refunds for shoddy service.

Best of luck.

Yeah, they are fairly useless from my experience.

I'd very much push for a refund or replacement, any decent bike shop should at least get you a replacement (and take up the faulty goods with the manufacturer themselves). Contact your bank if you paid on card.