So far I've been in touch with their support team, their customer service team, their returns manager. Their customer service said "Your query had been passed over to the returns manager to make the final decision. I can see that unfortunately this is all we can offer you, to complete your return. We can fully understand your frustrations in this matter and we do send our sincere apologies for the delays you have experienced."
So ignoring the corporate blah blah they are telling me to sod off - and these are the people responsible for good customer service!
I'm not sure how I can escalate it further with the retailer, so next stop I guess is the manufacturer for the warranty details, the European Consumer Centre for my statutory rights (I'm in Ireland), and maybe Trustpilot or something social to try and pressure them into doing the right thing. Any ideas welcome for how to escalate further