OK, done a bit more digging on this.
As far as the DVLA are concerned, you have, subject to certain risks, until the end of the month to tax the car, although officially they say they give you 14 days, in practice they actually give you the whole month.
As far as the Police are concerned you will not be legal to drive or keep your car on a public road. You may be stopped by Police, or given a ticket by police if they see your car parked on the road. (no point filling out a SORN for 1 1/2 days). There is no excuse as far is that offence is concerned. The DVLA 14 day grace is NOT RELEVANT to the criminal offence of using or keeping the vehicle without displaying a valid tax disc. But the Police, AFAIK (I'm sure Beemertastic can clarify this either way) will not and cannot clamp your car.
During the first 14 days of March they (DVLA) will not clamp it, probably won't clamp it in the remaining days of March either, but it's more risky that they might.
If clamped by DVLA during the first 14 days, you should point out to them that they say on their website they will not take action in the first 14 days of renewal expiry. You should be ok, and they should (technically) remove the clamp.
However, if clamped by DVLA after the 14 days, then you are in deeper doodoo, and you will probably have to pay a release fee and maybe a fine.
Sometimes Police Officers give more than just speeding tickets!
Itís hard to be fit as a fiddle when youíre shaped like a cello!