As Shero101 has indicated, a Statutory Off Road Notification requires exactly what it says, i.e. that your car is 'off road'. DVLA don't issue anything to replace the tax disk because it shouldn't need one when it's on private land. (Incidentally, it's actually illegal to use a car cover on a vehicle standing on the public highway because the tax disk is no longer visible. The law requires that it must be 'displayed').
Whether or not your car would need to be insured while parked on the public highway is also open to question. Unless there's a Crown Court (or higher court) precedent, it would come down to whether a particular magistrates' court regarded the vehicle as being 'used'. It's possible that you might still require insurance. See here:
http://www.theanswerbank.co.uk/Motoring/Questi on300657.html
Unfortunately, unless you can find some private land where you can keep your car, you may be forced to sell it.
Chris