Help! My tar driveway has a number of cracks, what is the best way to fix them?

Top Seal provided us with the following answer:

Cracks  less than 5 mm can be sealed best with QD Cement Sealer Compound which adheres well to Asphalt and is watertight and easily worked into the cracks. Wider cracks should be opened to allow for better/more product to fill. Asphalt may also be used on these cracks.

You can find more information on Top Seal Products as well as contact details for distributors in your area here.