Phoned the DVLA today as had the same Question, they informed me that if you tax your car online before the end of the month and display your old tax disc you have 5 working days grace, I then explained I had paid for tax online for my friend on the 28th of Feb and she was stopped on the 2nd of March and is being done not displaying a valid tax disc as this was within the 5 days grace is this correct, DVLA said that the police are aware of the 5 days grace period, but can still do you for not displaying a valid tax disc within these 5 working days its up to them.( well thats not on their website ) so beware its the chance you take.