Hi Charles,
as you are obviously aware in order to remove L plates and carry pillion passengers you must pass you practical motorcycle driving test.
The nest logical step for you is to take the Motorcycle Theory Test. (yes we know you have done a car one but the DSA are a money making organisation)
Next The Module 1 Practical Riding Test.
A skill test in an off road environment ( well a 140m x 40m fenced area).
Consists of serve and emergency stop at 50 kph. Slow speed ride, U turn and slalom.
Finally the Module 2 practical riding test.
About 35 minutes of on road riding with the examiner following you. A hill start and a couple of angled starts will be included in this.
The steps in logical sequence can be found here - http://www.begin-motorcycling.co.uk/index.htm
The Governments site on tests here - http://www.direct.gov.uk/en/Motoring/LearnerAndNewDrivers/PracticalTest/DG_178328