Bigdyer05

Is there any way to date my bike I bought it from a guy that had it from Belgium I haven’t registered it as of yet because I’m worried I will cop a load of tax on it for not doing it sooner 79,89,81 I’m thinking purely because of the style im mad about the bike so I’ve got to at some point go through the red tape to Reg it from what I understand I will need a letter from somebody who is authorised by dvla in a club to age date it then I can jump through the hoops thanks in advance Steve