So nice of the DVLA to prove for me why buying your tax online is unreliable.
Driver and Vehicle Licensing Agency
Extract from the link above
Due to a technical error, a number of tax discs ordered on Wednesday 24th January 2007 were delayed in dispatch. The error has been rectified, and all affected tax discs have been despatched, and should be received no later than 1st February 2007. Despite the delay, all vehicles affected will be registered as taxed on the DVLA system from the date of renewal.
I also just phoned the DVLA, 01792 782341, and they said, even under the circumstances you will find at that link (they are at fault), if you keep or use your vehicle on the public road you are liable to receive a fixed penalty notice, and this cannot be appealed, also, you have to allow 5 working days from the time of purchase for the disc to be issued (not received), work out the maths yourself.
I had a call back from the DVLA as I enquired as to why online buyers weren't informed of this on the website, and they say that as a fixed penalty for not displaying your tax is a "Police offence", they are not obliged, nor it is it their responsibility to warn those buying thier tax online that there can be problems, and that even though your tax in paid for, that the car should not be kept or used on the public highway unless the tax disc is on display.