Guys,
On your tax return (the back page of the employment section) there is a box (above the box for 'fixed rate expenses') called 'travel and subsistence costs'. It is well worth your while reading the notes referring to this box.
If you use your car for business journeys travelling to a temporary workplace, your costs are tax-deductable. In other words, if you are based at Manchester, but have to travel to, say, East Midlands, you can claim a mileage rate for the journey. Not to be sneezed at...!