Officially, you cannot tax a SORN'd motor untill the day of the tax coming into force, in your case the earliest day would be Monday 2nd March.

Vehicles which the tax is due to expire may be taxed 14 days in advance of the expiry date of the current tax period.

However, if you know the bint on the counter at the post office (I know the local one) then a little blind eye may be turned

Or alternatively just tell them that you want to buy tax before your current tax expires - they don't know wether it is SORN'd or not at the post office unless you tell them