When tax exemption was conceived by the Tories, it was designed to use a 25 year old rolling date to determine tax exemption. And this continued for a couple of years until the Labour Party got hold of the concept.

They stopped the rolling date and fixed the threshold at that in force at the time they got hold of it - which was in early 1998, so the threshold was set at the end of the twenty fifth year before that i.e. 31st December 1972.

It has generally been accepted that the last Stag built within the current tax exemption rules is LD21086. 

However we recomend that for cars with commission numbers between LD21087 and LD 21100, the owner should obtain a BMIHT certificate which will show the exact build date, as it will confirm whether or nor your car was built prior to 31st December 1972. If, on the other hand, your vehicle has a registration date recorded on the vehicle's Registration V5 or V5C, up to and including 7th January 1973, it will automatically qualify for RFL exemption, as the DVLA accept that the manufacturing industry traditionally closed down for business over Christmas periods, resuming the first week in any New year. Thereby confirming that it was impossible for any vehicle to be made between 24th December 1972 and 6th January 1973.

You simply apply at your local DVLA office, taking your documents along for amendment to tax class from PLG to Historic Vehicle. Do NOT apply to Swansea, they are not equipped for amendment to taxation class changes.