By the sound of it tbh your never too small to have an accountant. They'll for a fixed fee submit your tax return along with all expenses that youve had & others you didnt even know you can claim on .
As far as your materials are concerned they should have been submited as a capital asset on your balance sheet which are liable for deductions over a 3 year period for depreciation rather than a trading deduction that was for the cost of the materials at the end of the financial year. In respect of your 20% deduction you dont have to wait till the end of the year you can do it at any time but it may get a bit messy on paper- i'd proberbly wait now to april & start a fresh....some like myself do the same as you & deduct the whole lot- it reduces your instant cash flow but makes for a great tax rebate-personal preferance really but again tbh if your asking these type of questions although the forum can give u insight & ideas i would strongly recommend a local accountancy firm, & depending on your trading status depends on what you need.you may just need some advice & a book keeper as a 1 man band or a chartered acct if your ltd but its all tax deductable .
As for you joe lol the carpul tunnel & body wearage arnt deductable bout any treatments , medicines or bandages are incl private physio