Self assessment declaring losses (rent Arrears)

Hi Just wondered if somebody could let me kow im comleting my self assessment for tax year ending 2012

Our Tenants had to be evicted with approx £4000 rent arrears at the end - I have completed the return with the losses incurred due to repairs and replacment - (they left the house in an appalling condition) but is there anywhere you can declare a bad debt ie the rent arrears on your property although i am making every effort to try and claw them back i dont see them as recoverable ?

Also as a seperate issue I have read somewhere that you can claim an allowance for using your home as an office approx £4 per week without providing evidence of your outgoings is this still in effect do I have the amount correcnt and can i claim this if im using an letting agent?

Thanks for your help