I'm in exactly the same situation. Probably picking up a saph cos on Tuesday. It has no tax and is currently uninsured so the present owner can't tax it.

It has a full MOT and I will obviously be insuring it on the day if I buy it, but won't have the cover note.

Will the insurance be put onto the national database straight away? Do you need the cover note if you go to the post office or do they do they use the computerised system now?

Cheers