In reply to Bjartur i Sumarhus:
Any mention of court makes most people very cagey and defensive, in your position I would work out what I wanted (i.e. refund/replacement) and just ask for it, explaining why I thought it was reasonable. Only if they started to try to fob me off or tell me I'm not entitled to what I've asked for would I cite the Consumer Rights Act or similar and if that still didn't get anywhere I would then produce a formal letter (pre-prepared, just in case) stating my case and giving them 14 days to cough up, I wouldn't even threaten court yet. If that didn't do the trick, and if I was sure they were acting contrary to my statutory rights, then and only then I would start a claim. If you want to go in all guns blazing though, that's your choice.