as focus v8 says, if you get spotted driving it without tax they will throw a fine your way BUt you can plead that youwere collecting the car and were unaware it had no tax on it, and by the time you figured it all out it was too late

as long as you can proove to a copper who might stop youthat everything else is all above board (they'll probably give you a producer or the like) with the greeen bit of the log book you should be ok driving it back today, but i wouldn't be driving it until at least the cover note turns up

as for the 14 day leeway, that was back in the day when everything was done by hand and not by the big black box on the back seat in the adverts

asking the seller to get the tax for you might also be a problem if the car isn't insured