Mileage charged to clients

I charge a flat mileage rate to my clients, which is added to the clients invoice. My question is, when declaring my turnover for my tax return (Sole trader) do include the full invoice amount including the mileage or do just declare the total amount less the mileage. Very confusing but cant find anyone that can give the right answer.

Hi Chappy,
You include the full amount of the invoice as part of your business turnover and then deduct the mileage as an expense.
Thank you.