Thank you for the information. As stated, this is not a live chat but an email service, however I apologies for the delay and will deal with this for you now so you have an answer in the morning.
Depending on how long the have had this as a "live" issue, you may be very correct in the fact that they have ran out of time to claim this as there is a 6 year limitation as you are aware. If they have exceeded this limitation, it would constitute a legitimate defence on your part. If you are not aware of how this overpayment has came about, you should ask them to provide information about this. If you dispute this or feel they have not made the claim in time (over 6 years) you will want to appeal this decision. If they refuse the appeal on any grounds, at this point you can look at taking the matter to a tribunal to obtain a legal order on the matter. You can of course either chose to do this yourself, or you could look at instructing someone to represent you. However, what I would highly suggest is that you go down to your local citizens advice (you do not need an appointment). Usually, the bureaux have benefit experts who, whilst not legally trained, have a very good grasp of the legalities of the benefit system. They provide a free service and can not only assist with your appeal, but can in some cases offer representation at the tribunal should it require to go to that point. If you win your claim you can potentially claim back what you have paid out in overpayments. Obviously I do not know what their defences would be but your representation would be able to advise on that once received.
I shall provide you with a helpful link that will assist you in finding a solicitor/representative near your local area. This will provide you with someone nearby your area who can assist you if required.
I hope this information proved helpful. You will find a local solicitor who deals with these matters on the law society webpage which is as follows;
https://www.lawsoc-ni.org/solicitors (N. Ireland)
Hopefully, I have answered your query in a way that is simple and easy to understand. If anything remains unclear, I will be more than happy to clarify it for you. In the meantime, thank you once again for using our services.
Should you require any further assistance on this matter, please do not hesitate to post a further questions for additional assistance.