"I purchased my CO alarm about two years ago. It packed up so I needed a new one. I read the review for a new one on the Screwfix website and was depressed to read that review.
However I rang the company and had the leave a message commentary. No difference.
However they did ring me back about two hours later. Asked for details and i was able to provide the codes which proved it was within the 5 year warranty. They promised to replace it and a few days later the new one arrived at no charge.