Know what you mean Brian
The system can catch you if you don't do things in the time scale allotted. Where I'm coming from, by mutual agreement I purchased a car from a relative with the view of him losing the minimal amount of tax and me not paying for more than I had to. Timed it for the transfer date to be the 29th of the month. Documents sent and for a 10 day period, the online tax check kept stating that the car was taxed. Didn't want to risk the DVLA having more dibs at our money than what they were entitled. Then bang, registration got changed to our name and the online tax check status changed to Untaxed.
The system is that new owner is required to quote the 12 digit V5C/2 document ref code. As I found out this is only valid whilst the vehicle is still registered with the previous owner. Once the DVLA press the transfer button, the new owner can't do an online vehicle tax. From there, the new owner will have to wait for the new V5C document to arrive with the new different doc. ref. codes. The way round it is to tax the vehicle at the Post Office. During that period you are at risk of getting done for an untaxed vehicle if caught on the road.
If left off the road untaxed, in theory, can they do you for not doing a SORN?