Here is the appropriate section from the operational guidelines on Means Assessment:
Any cash income from the portion of a house that is not personally used or enjoyed, is assessable against the claimant as income.
....If rooms are let furnished, 5% of the gross amount received are allowed for wear and tear and 15% of the gross receipts maybe allowed for voids (i.e. periods when accommodation vacant between lettings).
From reading this, the amount of rent assessed should be the actual rent received less 20%. Check that this has been done on your means assessment.
You may qualify for Mortgage Interest Supplement - have you applied?