The 1/3 is acctually wrong. If you work from home and have an area, say a bedroom, that you mainly work from, then you can claim £4 per week without having to produce bills and receipts. If you want to claim more than that (it sounds like you don't) then you have to supply supporting evidence.
http://www.hmrc.gov.uk/incometax/relief-household.htm
I would claim it. If you don't claim it now, but want to reduce your tax in future years by claiming for it, it could be challenged. Better to claim it now and be consistent. If you claim £4 x 52 = £208 then it is easy to do the form.