I can't recommend the webinars or workshops run by HMRC enough. It is important that you understand what you can and cannot claim for and with all due respect, an accounts book from the wholesalers isn't going to give you the understanding you need. Start by looking through the info on the HMRC website and take it front there. Also, it's worth noting that many accountants will do initial consultations free of charge and all accountancy fees are a deductible expense so if you're not sure, they will almost definitely save you more money than they cost. Good luck.