There's no 'proper' amount for mileage. An employer can pay UP TO 40p per mile (for the first 10,000 miles - and then 25p per mile) without HMRC regarding the payment as taxable income.
However the strange way that HMRC's system works means that they start by counting every single penny you get from your employer as taxable, then knocking off 40p per mile to find out what you must pay tax on. Since you only receive 25p per mile, that means that an extra 15p per mile should be taken off your taxable income. Assuming that you're paying income tax at the standard 20% rate, that means that (if you're not already getting the relevant tax relief), HMRC will owe you only 3p per mile, not the 'missing' 15p per mile which your question refers to.
Contact your local tax office for the relevant paperwork:
http://search3.openob...or/locator.jsp?type=1
Chris