You must have DVLA acceptable evidence linking the chassis/frame number to the orig reg'n. E.g. The buff log book, original bill of sale, or a certified copy extract from the local registration office's records.
Old tax discs on their own are no longer acceptable. I've never applied for an orig reg'n for a 'young' vehicle which needed MOT's but I assume they show the reg'n and the chassis number. Maybe old MOT's are acceptable if the reg'n hasn't been re-issued by the DVLA. You can check that online here: