You need to keep all receipts of what you buy for your business.
All out going invoices for jobs,proof of payment from customers,if you run a software package this will have details of when paid by customers.
Paying in book for bank,for paying in,cheque books for paying out.
If you get an investigation,they will reconcile your software with you cheque book and paying in book,if that all adds up not a problem,if you do otherwise with payments ie cash and not bank it the sums won't add up and you will be int the poop and in trouble and yes like others say you need an accountant a chartered accountant is qualified and cannot afford to cook the books if he does he's I big trouble the the tax man and can be struck off,
If you choose to use a NON CHARTERED ACCOUNTANT and a tax advisor,anyone can be a tax advisor as they are not qualified,you could be in the poop if you get an investigation.