[What year is mine]

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