i have brought a professionaly converted lpg mpv (now duel fuel),
complete with all documentation.
Its still listed on the log book as petrol.
what do i need to do to get the (slightly) lower vehicle licence? Is it
worth doing?

Hi mmzz I have changed my Astra 1.6 estate to alternative fuel and saved myself £10 on the road fund license, . I changed by going to my the local DVLA office, but you can do it by post. If you go to this site Changing your vehicle details on your registration certificate : Directgov - Motoring
it will explain what to do, you will need a current MOT, insurance and proof of modification. i hope this will help mike