Tax class changes to "Historic" processed only when a licence application is made, not when a SORN is made
As very nearly all MGBGTV8 models are now eligible for road tax exemption, most owners will want to arrange for the tax class change from from PLG to Historic as soon as the opportunity arises in April as the eligibility for the 40 year rolling VED exemption arrives. So if your car is approaching that point and is currently on a SORN then you will need to apply to tax the car at the current VED rate before making the tax class change to Historic. Following a successful the tax class change you will need to make a road tax exempt application for for a NIL value VED and seek a refund for the complete months of the remaining period of the "paid for" VED.

Tax class change guide and flowchart
Our guide and flowchart will be updated in the New Year for vehicles built in 1975 where eligibility for the tax class change will apply from 1st April 2016, subject to the Chancellor including the rolling 40 year exemption in his Budget measures and then in the Finance Bill in 2016. Guide & Flowchart
VED info gateway

Posted: 151021
Earlier this year a member reported a case where he had purchased an MGBGTV8 and put it on SORN and at the same time applied to have the car transferred from PLG to the Historic class for road tax. He later got an MOT and attempted to tax the vehicle at a Post Office. The counter clerk however refused to so, quoting the latest edition of the leaflet INF34 which states “The first time you tax a vehicle in the historic vehicle tax class you must apply to DVLA, Swansea, SA99 1DZ.“ The query was passed to the FBHVC (Federation of British Historic Vehicle Clubs) saying "do we know whether this literal interpretation by the Post Office counter clerk is the intention of the DVLA or have they written the leaflet for the vast majority of cases where the first taxation in the historic class is because of a transfer into that class from some other class, and not considered the position where a vehicle is already in the historic class but has not been taxed in that class because it has not been taxed at all, being on SORN?"

After some delay, which DVLA admit was because it took a little while to sort out, the FBHVC has received the following reply from DVLA to their question which combined the query above and two others queries concerning change of tax class.

“If the pre-75 date is held on the vehicle record the change of tax class can be carried out by the Post Office and the INF 34 is now being revised to reflect this. A change of tax class can only be carried out when a licensing application is made, not when a SORN declaration is made. However the Post Office can re-license a vehicle in the Historic tax class if it has previously been under a SORN. Where a vehicle has to be transferred from the obsolete tax class “Exempt” the Post Office system will allow the change to be processed. All applications from other obsolete classes would have to be sent to the DVLA.”

Comment: before a successful tax class change from PLG to Historic, the DVLA records of the "date of the vehicle" will be the date of first registration and not the build date because their records do not hold the build date, only the date of first registration. This is an important matter because a car might have been built in December 1973 but not registered until June 1974 for example - sales of MGBGTV8s were slow at that time because of a fuel crisis in the Middle East. So a key part of a tax class change application is submitting evidence of the "build date" of your car particularly in cases where the build date and date of first registration straddle the year end as in the example noted above.