Sorry to hear of your probs....Buster accessories have lots of Duke regulators mentioned on their website for £60 but sadly, not yours specifically.
Dont know how good they are or if they need mods...here is a link anyways (look in the spares section)
http://www.busters-accessories.co.uk/ If it is the regulator try taking your old part to an electrical motor factors (locally I used a firm called Diesel auto electrics who are excellent and sourced me a new started relay for £10 !!!)
Good luck