Hi there, the process for changing your V5 details with dvla is surprisingly simple yet somewhat 'long winded'
The new details, engine number, cubic capacity and fuel type need to be amended. Fill in the relevant parts of your V5 and return to dvla Swansea but keep a photocopy and a note of the date you posted the document, just in case

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You will receive a letter back, complete with an envelope addressed to a specific department that seeks to confirm your alterations, they do not trust you!
This return letter will ask for some supporting evidence of your conversion, unfortunately a new mot certificate will not suffice as the fuel type is no longer recorded on the mot certificate
The simplest solution is to return this with a supporting letter in the addressed envelope from the garage that carried out the conversion, or a local garage that is willing to confirm the details of your conversion.
Do not be tempted to include the supporting letter in the original application with your amended V5 as it will be ignored. Dvla state that amended V5's that are sent to them should not include any other paperwork in the envelope as, and we have found this to be correct, all other correspondence found in the envelope with the V5 will be disposed of, or at least ignored!
When you send the second application, together with your supporting letter, it will be in the addressed envelope supplied by return of post and reach the correct department and be processed accordingly.
Once you have received your amended V5, you will be able to contact tfl and send them, also to the correct department, a COPY of your amended V5 and a copy of your supporting letter from the garage, to ensure that they amend their records.
Hope this helps all of you to get compliant!
TIM