Mike,
You need the same paperwork to get anything registered. I didn't have a release form for the Ferret or Fox, but was able to get dating evidence from the Tank Museum. Without seeing what dataplates are in the Saracen it's hard to know what you'll need to do, but it should be possible to date it from the contract number if this is on the plate. You will need a letter from the MVT/Tank Museum etc and you can present this along with the insurance, MOT (or exemption) and V55 in order to register it.
Chris