As far as I'm aware (from memory?) there is no 'mileage' allowed for Sole Traders. You can only claim a % of the running costs of your private car instead.
This makes it somewhat easier, as you don't have to get the % exactly right down to the last mile. You just need to work out roughly what you think it is, but staying within reason. You should keep all receipts from your private car (finance repayments, insurance, petrol, services, repairs etc) and base your figure on these.
You would only need to calculate it at the end of the year and put it as an expense against your year end Income Tax returns.