I presume the OP was referring to Rent Supplement under the Supplementary Welfare Allowance Scheme and not Rent Allowance for tenants affected by de-control of rents. The former is commonly known as Rent Allowance and causes a lot of confusion.
I would suggest calling your Community Welfare Officer (CWO) to find out specifically how your Rent Supplement (Allowance) will be affected by you working as it's a very complex area and it depends what type of social welfare payment you are on, standard rules, retention rules, how many hours you intend to work, how much you'll be earning, etc. Utimately it will be your CWO who will make the decision about the amount of your Rent Supplement/Allowance so it's really in your best interest to approach them.
I'm also including a link to the INOU's excellent publication called 'Working For Work' which gives loads of information but it's not currently available online. [broken link removed]
Perhaps you could request a copy from them or your local Citizen's Information Centre.