The company you do work for must register you with hrmc first then they by law must deduct your 20% and pay you minus the tax, they must ( but try not too) give you a receipt to prove they paid it.you keep theses along with fuel, tyres, servicing etc , tools, boots, clothes, ppe.any receipt for new vehicle purchased.any computers , ink , paper , printers etc , your wife can get you a bit of tax off because she types out your invoices etc. if you get a decent accountant they will sort it and you don’t always need receipts for certain things as thiers an annual allowance allowed. Child tax cred