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Thanks for your question.
If you bought the sofas as a consumer the standard of quality applicable under statute to the sofas are that they should be of "satisfactory quality" (s14 Sale of Goods Act 1979). If one of the arms is damaged after 5 months after you have used them in only the usual ways one would use a sofa then this is a breach.
You would have to give the supplier an opportunity to repair the arm first. If they do this and the sofa is returned to satisfactory quality then you will have to accept it. If the sofa is returned to you following repair and it falls below this standard then you will be able to ask for a replacement of the sofa that is damaged. You will not be able to ask that both are replaced I'm afraid.
If this happens and you a convinced the sofa falls below the standard above then you can ask a local solicitor to send a "letter before action" stating your intention to litigate unless a replacement/refund is furnished. This would cost around £40 +VAT. If this does not get any traction then you may be forced to litigate via the small claims court: http://www.moneyclaim.gov.uk/
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