You're entitled to a repair or replacement from the retailer if the problem is due to an 'inherent fault'. (i.e. something which was actually wrong with the set at the time you bought it, such as the use of poor quality parts or poor manufacturing standards).
If the problem had occurred within 6 months of purchase there would have been an automatic legal assumption (unless there was clear evidence to the contrary) that the problem was due to an inherent fault. Thereafter you have to be able to prove that it was an inherent fault which is the cause of your troubles (or, at least, to be able to show that it's the most likely cause).
In theory that means that you'd need to pay for an independent expert inspection of the set. However simply challenging the retailer to suggest an alternative reason as to why the set might have failed could be all that's needed.
Forget the staff at your local Co-op branch and contact their head office instead. You'll probably find that they're sympathetic and will sort things out for you. If not, start dropping hints that the local press might well be interested to know about their appalling service - that usually does the trick!