according to hmrc
How HMRC deals with a company benefit in your tax code
HMRC adds the value of the benefit to any other untaxed income you have and takes away the total amount of income you've not paid tax on from the total value of your allowances and reliefs. The amount you're left with is the tax-free income you are allowed to receive in the current tax year.
Example
You are entitled to the basic Personal Allowance of £7,475 (the amount for the 2011-12 tax year) and your employer has provided you with private medical insurance worth £600:
your total allowances and reliefs are £7,475 (your Personal Allowance)
your total 'deductions' are £600 (the value of your medical insurance benefit)
your total tax-free amount is £6,875 (£7,475 less £600)
So, you have tax-free income of £6,875 and your tax code is 687L.
HMRC will usually send you a 'PAYE Coding Notice' that explains exactly how they have dealt with the benefit in your tax code.
so the rest of your wage will be taxable at either 20 or 40%
http://www.hmrc.gov.u...-company-benefits.htm