My experience, done yesterday (27 January 2022).

My ex-car final renewal - February 2019 (road tax & insurance expired on February 2020) - NCD 38.33%.
The car was left abandoned since Sept 2019 until I sold the car in April 2021.
Notified my agent that I'll purchase a new car before end of 2021.
Managed to purchase a second hand car in July 2021, done the ownership transfer on 26th January 2022.
My agent managed to transfer my old NCD (38.33%) from the old car to my 'new' second hand car, all done yesterday.

If I didn't purchase or buy a new car, the NCD percentage will be reduced on year-by-year basis, based on the final NCD percentage.