Claiming mileage

I used to do this in the past but not done if for many years. I understand it's still possible to claim back mileage expenses up to the rate of 45p from HMRC. Is this done annually and I can't remember if it's refunded in the form of a cheque or your tax code is amended. Also is this possible if a car is available it you choose to use your own or if the company pays a small contribution.