When can I apply for my Triumph Spitfire to have historic vehicle status?

I own a Triumph Spitfire that's just over 40 years old, registered on 14 February 1978. When do I apply for Historic status, now or after 21 May 2018?

I own a 1971 motorbike with a lapsed MoT - can I ride it without an MoT when the 40-year exemption begins?

I own a 1971 motorcycle. It was insured last year in April, taxed (historic vehicle class) from 1 June and MOT'd until August 2017. I never got around to using it at all last riding season, so it remained...