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You need to register with the tax man, that you are doing some self-employment work.
You can claim the same things that all self-employed claim.
Because you are already in employment you will have used you personel tax allowances in this job, so any self-employment profits will be taxed at the going rate, you will also have to pay class 4 national insurance on all profits as well, which is 8% on all profits up to £34840 then 1% on amounts over £34840 at the moment.