Advice needed for calculating expenses (UK Tax)

[Julie4]

Hi. I'd really appreciate some advice on how to calculate expenses for letting out my room through Airbnb for  UK tax purposes. I've found the guidelines very woolly. I expect that my earning will go over £4250 for the 2015 tax year, so need to calculate what I can allow for expenses ie laundry, cleaning, bedding, bills, repairs etc this is what isn't clear to me and do I need recepits (as I haven't kept them). I'm using the Rent a Room Scheme as guidelines. Thanks, Julie