Evening Big Truck. An easy formula to work out how much the total price is less the Vat is multiply 7 divide 47
i.e. your figure of £0.785 / litre equates to £0.12p / litre VAT. In other words you would be paying £0.67p / litre. (ex. VAT)
Having said that remember that you would still be paying the VAT up front on all your purchases. As Jonathan has pointed out 3000kms is more than 1500 miles. My maths make it 1875 miles. The maths are the important thing of becoming an owner driver. Many haulage companies work on a profit of around 3-5% so its critical that you have your figures right.
Litres to gallons — multiply 4.546
Gallons to litres — divide 4.546
KMs to miles — divide by 1.6
Miles to KMs — multiply by 1.6
The other important thing to consider is your cash flow. What are your terms of payment? Remember that if they are 30 days that will mean that you are waiting two months for any income. i.e. if you put it on the road April 1st you will bill that on the 30th April and get paid for it around the 30th May.
Hope that helps a bit. I wish Trucknet had been on the go years ago when I went into it. It might have avoided my bankruptcy in '96 
Best of luck
Phil