Yes you are supposed to declare it on your tax return, (self Assessment). As rental income is deemed as taxable income by The Inland Revenue.
If you are PAYE, then you would need to contact the Inland Revenue and request a self assessment form, where these details can be added and submitted in the new tax year, which started April this year. You usually have until December 30 to get your tax return in. It can also be done online.
I don't think it has anything to do with HMCE though, they deal with VAT. And yes you can claim back expenses, for any furniture, renovations etc.