The property income allowance is only applicable if your expenses are lower than £1000.00 as you can only claim one or the other. If your expenses are higher than £1000.00 you cannot use the allowance.
Can this change from year to year i.e. use the allowance one year and not the next ? do you still enter the actual expenses incurred and claim the allowance? or just claim the allowance without entering any expense figure ?