If you contact BMIHT they can (for a fee) provide you with a certificate confirming build date, chasis numbers any special order and any other info they have. They do I believe do a simple letter if you just want confirmation of a 1972 build if applicable. Just put BMIHT into Google and the site complete with telephone numbers comes up.
Their certificate is acceptable proof of a 1972 build date for DVLA. As a rough rule of thumb if it was registered in or before March 1973 it may well be made in 1972. Anything registered later don't get your hopes up too much.
Hey dont be such a tightwad ! You should feel proud and priveliged to be able to stump up yet another unfair tax. You can hold your head up high while so many of us can only feel so guilt ridden that we are unable to contribute !