If the gross income is less than £4250 then you will not have to pay any tax, so long as the room is in your main residence - if jointly owned then £2125. You still have to put an 'X' in a box on the SA return.
You can opt out of the rent a room scheme, if for example you have made a loss.
If you have income over £4250, then you will have to pay tax in the normal way.
I had a quick word with an accountant friend and he confirmed what you guys are saying
P - Just for further clarification ( I forgot to ask my friend this.) You say that I should cross box 4 on the property supplement. Presumably this means that I should add the rent a room income to the general property income and put the whole lot in Box 18 Then net off the exempt amount in box 35. is this correct
If the income is less than the 4250, then you only place the 'x' in the box to tell HMRC that you have rent a room income which is less than the threshold. You would not show the actual income anywhere.
If you have more, you just add to the total income and it is taxed in the usual way - I don't think you tick the box if the income is more.