I beleive you need to take your CBT (compusory bike test) or something like that. That lasts 2 years in which time you have to take a full test. Also check you have a provisional entitlement for motorcycles on your license.
Yes Orangesauce1 is correct you do need to do your compulsory basic training to be able to ride a scooter or motorbike of upto 125cc. This lasts for 2 years in which you can just re-take your CBT to last another 2 years or think about taking your full bike test.
If you obtained your car licence before 1st February 2001 you do not need to complete a CBT to ride a moped (engine not over 50cc with maximum design speed not exceeding 50km/h - approx. 31mph).