Just in case anyone doesnt know I just came across this on the DVLA website -
If you buy a vehicle that already has a SORN from its previous keeper, the SORN will finish as soon as you buy the vehicle. You will need to make a new SORN.
Penalties
If you don’t make a SORN when you have to you’ll automatically be fined £80, will need to get a new tax disc and must pay any tax arrears. You could also get a County Court Judgement against you, be fined a minimum of £1,000 and be liable to prosecution.
I found it by accident (having bought a second bike which was SORN and is staying that way for a few months) - SO glad I did.