Rent Supplement rules generally exclude certain categories of people (those working more than 30 hours, full-time students), and the statutory limits that payments are based on are much lower than typical market rent values. However, there is a wide discretion (known as Article 38 discretion) that allows the normal rules to be set aside and for Rent Supplement to be awarded in exceptional circumstances.
If you think that you may not be entitled to Rent Supplement for one reason or another, or if your payment has been based on a lower rent limit and not your actual rent, you can ask them to exercise this discretion. If they refuse, you can appeal any decision to the Social Welfare Appeals Office.
If you make a reasonable case that you need the support (or more support) with supporting evidence, they should at least listen to you. Make sure to use the word "discretion" in your dealings with them. This gives them the flexibility to deal with your case, but also subtly sets up any refusal for possible appeal.