Thanks - yep, had seen HMRC ones but was still in the dark really about whether % is allowed or not.
Now in my own case, the rooms (I'm assuming I exclude bathroom, kitchen and hall?) were pretty much 25% each of floor space anyway so is the claim still 'legit'?
Final question - with this then, do I need to upload a spreadsheet? Also I I need to attach documents e.g. rental agreement, breakdown of bills?