This link goes to a commercial website but the information provided is a good guide to allowable expenses:
www.just4let.co.uk/media/pdf/Landlord%20Guide%20to%20Tax.pdf
So start with the total income from the flat. Deduct all permissible expenses. Split the resultant figure into two equal parts for the tax forms unless that would still leave you with part of your personal allowance unused. (If so, split the income so that you use up all of your personal allowance).
Chris