Basically to Sorn a car it needs to be off a road, ideally on your own private property.
If you live in a block of flats and you leave your car on private land, such as the flats car park, it technically not your land and someone could report it and it can be fined or taken away. So I would not chance that, best thing to do is tuck it onto a friends drive if they have room and do not mind or get a garage to store it away.
Once your vehicle is sorn and off road it no longer requires insurance, but if you wish you can still have it insured even when sorn.
If your tax runs out and you haven't declared it sorn the DVLA will usually write a letter to you within a week or two, usually they give you a chance to sort things out ASAP with discretion or they will fine ya a$$.
The new rule in april that you are on about states if a car is left on a public road it needs to be insured, even if its got tax it still needs to be insured, otherwise the owner would have to sorn it and take it off road, as people will just get a car, dump it outside and never work on it etc...even though its taxed but not insured.
You can drive the car right till its final MOT expiry date, I've done this on a few occasions, but I'd always make sure its ready to pass the MOT if I do this by popping to my mates garage and doing a MOT checklist with him.
If any information is incorrect I'm sure people will rectify it
