I've had good success with Fiebing's Professional Oil Dye, in black. It's actually intended to be used as a leather dye, but several places that sell guitar-making supplies, such as Stewart-MacDonald, carry it as a fingerboard dye.
It takes a while to dry, so be prepared to be wiping black smudges off your fingers for a few days. Don't know about availability in England.
http://www.tandyleatherfactory.com/h...ture=Product_1