The Driver & Vehicle Standards Agency have published a revised form VTG 788 to record HGVs with DVSA.
You will need to use the latest version of the correct form depending on how you registered the vehicle with DVLA. It only needs to be done once for each relevant vehicle to allow you to book an annual test for it.
VTG 788 This is for vehicles under European community whole vehicle type approval or National small series type approval.
VTG 789 This is for a vehicle that has an individual vehicle approval (IVA)
These forms should be completed after you or the dealer registers your HGV with DVLA . There is no fee to record or change the vehicle details with DVSA.
Source: Driver & Vehicle Standards Agency