Hi Arrow.
All bikes must either be taxed or SORNED - but vehicles constructed before 1st January 1973 are eligible for free tax. i.e. you still have to get a tax disc, but it is free of charge if it is made before 1/1/73. The post office will be able to advise as and when you take your tax form in. Obviously you still need the usual MoT and insurance too. Hope this helps.
Cheers
Nigel.