I have got to the bottom of it.
Yes the bike has to be 40 years old but it can't be classified as a classic until the first January after the bikes 40th birthday. In my case the bike will 40 years and 4 month old.
It can't be done online but just take you insurance, log book, pictures to show that it. has not been drastically altered to the Post Office and they do it there and then.