You can claim 45p for the first 10,000 business miles and 25p thereafter together with an extra 5p per mile for each extra business passenger per trip. The detailed instructions on this can be found here.
You must keep a detailed log of business mileage for each business trip in support of the claim. A spreadsheet for this can be downloaded by click this download – Mileage and a video explaining how it works is here.
For employees claiming mileage the employer can also reclaim the VAT on petrol and maintenance but you will have to retain receipts in support of this.