The new system requires a vehicle owner to declare their vehicle off the road in advance by completing form RF150 Declaration of Non-Use of a Motor Vehicle.

Purchasing a car:
So if you purchase a car, You have 10 days from purchase (Even if previous owner has declared it, It's cancelled when it's sold) to either tax it/declare it off the road with an RF150.

Tax is only payable from the 1st of the month in which your purchase it, You do not have to pay arrears if your only after purchasing it once you get it taxed/sorned within 10 days. (Previous owner is also not liable for arrears)

Re-Taxing during SORN period:
You can cancel the declaration at any time by renewing your tax in your local motor tax office. The tax will commence from the first day of the month in which you tax the vehicle.

Before SORN expires:
When your SORN is about to expire, You should get a renewal in which you can either re-tax the vehicle or declare it off the road for a further amount of time. Just call into/post the RF150 to your local tax office