I did get a recent update from DVLA on this. Posted on another thread.
If the taxation class is not already shown as 'Historic Vehicle' on the V5C for your vehicle, you will be able to change your taxation class at the point of relicensing at the Post Office®. The V5C must show a date of registration prior to the 01.01.1974. If this is not recorded on the V5C and the vehicle was manufactured prior to this date you will need to produce dating evidence and apply by post to DVLA, Swansea, SA99 1DZ.
Acceptable dating evidence can be obtained from:
• An extract from the manufacturer/factory record - these will have the chassis number of the vehicle against the month and year of production*
• An extract from the 'Glasses guide'- which is a comprehensive guide on production dates and includes chassis numbers
• If the vehicle has been imported and already registered, the original foreign registration certificate.
*A certified copy of the factory record (with the embedded stamp) will be acceptable only from the British Motor Industry Heritage Trust (BMIHT)
The keeper will need to write ‘Historic’ in the change to taxation class field in section 7 of the V5C and sign section 8.
If the application is successful, you will receive your tax disc within 2 weeks.
An updated V5C will be returned within 4 weeks. If a V62 form is submitted a V5C will be returned within 6 weeks.
So acording to thier own information, the vehicle should be changable on the VL5!