Been doing some more thorough reading on this, and I find that I was wrong about refunds.
In fact from 1 Oct you can tear up any tax disc you have, as it is all on the electronic system and there is no requirement to display any more.
If you sell your car you will automatically get a refund of any complete months, and the new owner has to buy new tax from the beginning of the month.
As I have just sold my softdash (cheque clears today), I shall declare sorn from Friday. On Sunday it will be picked up by a transporter, and I will then do the V5 for transfer of ownership. He can either tax straight away, or leave the sorn in place until he is ready to put it on the road. Quite convenient timing.
(It doesn't need transporting, but as it is going 300 miles it actually works out quite reasonable re train fares, hotels, and 2 days to collect it!