Mileage rates for expenses (AMAP)

The amount you can be paid for business mileage without reporting to HMRC is at the rate of 45p for the first 10,000 miles and 25p from then after.(re 2011/2012 tax year)
There are special rules for business miles classification and if you exceed these rates then National Insurance contribution (NIC) may be payable.
Essentially business mileage is that which is for travelling from one client to another or to and from a temporary business place of work. Not from home to a permanent place of work.