No need to SORN unless the tax has run out but don't drive it (unless you're going to an MOT test) 'cos if you have an accident your insurance company will probably disown you - and the police will probably come down on you like a ton of bricks. They're getting very intolerant of uninsured drivers.

If the tax does run out and you don't SORN it immediately I believe they automatically issue a fine 'cos the vehicle has to be one or the other.