I have had RD400-1 SORN since I the old tax ran out in January. I just went to tax it and the DVLA site informs me that since the bike isn't SORN and hasn't been taxed since January I might get communication from the enforcement people (I am sure that means a fine). I am sure it was SORN'ed so I check back through my emails and, lo and behold, there is an email confirming the bike was SORN'ed on 29th January.
The road tax website has a helpline number so I call it, after much pressing of numbers to make sure I get through to the right person, a message telling me the call will be recorded and many messages to tell me they are very busy I get through to a person. I explain about the bike and the SORN and offer to read out the reference number - but she doesn't have access to that screen so there is no point, I am then told that because of a new keeper the SORN ceased. I am the new keeper!
Although I became the new keeper on 14th January, I SORN'ed the bike on 29th January and the tax ran out on 31st January, because the last keeper didn't send the docs off or the DVLA didn't get around to registering me as the new keeper until after I had SORN'ed the bike, the SORN stopped. I am sure that if I had left the SORN until I received the V5 they would be sending the boys 'round for no tax in February.
If they think they are going to fine me they are sadly mistaken.
I am not sure what this means in the new system. You can't currently tax a vehicle until the V5 is in your name. You can't use a vehicle on the road unless it is taxed. If you SORN a vehicle and they haven't changed the details on the V5 then that will stop as soon as you are the new keeper. Tax can not be transferred under the new system.