As an employee you will only be taxed on a company van if you use it for private (non business) journeys. Private use other than commuting (work to home and vice versa) means using the van for your social activities or your supermarket shopping.
If your employer allowed you unlimited use of the van, this will be assessed under PAYE as benefit in kind of £3000. If your employer also pays for the private fuel, there will be additional £500 benefit in kind on you. If you are basic rate tax payer (20%) for the tax year 2009/10, your tax bill for private use of the van will be £700.
Where your only private use of the van is for home to work travel (and vice versa) this is not treated as private use so you will not be taxed under these circumstances if all other private use is not allowed by your employer. Your employer should ask you to sign an agreement stating that you will only be allowed to use the van for private purpose to travel from your home to work and vice versa. Any other private use would not be allowed. You also need to keep mileage logs recording your mileage for each business journey and the reason for the journey.
To ensure you do not end up with an unwarranted tax bill, please make sure you that you sign the only business use agreement before the new tax year starts – 6th April.