Put the key in the ignition and without actually starting the car, turn the key forward, then back. Do this 3 times within 5 seconds and the 3rd time keep the key pointing towards the windshield. if there's a code stored in the computer it will show up in the center dial, example: P0700 and if there's no codes stored it will say "done".
Also, if you haven't done, take a look at a great U.K. site that has loads of members willing to help out with questions. You don't have to join but some of the forum won't be accessible if you don't.
UK Cruiser Club :: Index