psalliss, don't get too edgy about Income Tax on this income, because from what you've said, it's VERY likely you won't pay any.
The Revenue operate a scheme called the Rent-A-Room scheme, designed specifically for people letting a room in their house, rather than professional landlords, and providing you meet basic criteria, the first £4250 of such rental income, per year, is tax free.
Get yourself over to the Revenue website, check out this scheme, make sure you qualify and make sure you notify the Revenue of anything they need to be told .... but don't worry to much about this getting taxed. It shouldn't.