Hi --- Under the sale of goods act you can take it back to the Co-op.... your contract is with them as the retailer and the goods must be fit for the purpose which they were sold, which they are obviously not, therefore you could demand a refund in full or a replacement set.
They should provide a replacement set if it is under 6 months old the law presumes it was faulty at the point of sale unless they can prove otherwise, if it's over 6 months old they can offer to repair or replace if the repair is economically unviable.
You do not have to call out an engineer that is an extra statutory right that is your choice, demand they take appropriate action as above, under the sale of goods act.