Not sure to be honest. Back in the day, I had a car off the road for about 5 years and never had to SORN it after the initial time of doing it, but I guess the rules have changed.

First time I declared the Escort SORN I assumed I just had to do it the once, and did get contacted about it around 15 months later saying I hadn't renewed it, and had about £95 to pay, but I got out of having to that, but can't remember much more.