Had a post card from the DVLA this morning and this is the first paragraph
“From 1 September 2008, if you pay for your vehicle tax by phone or online before the current tax disc runs out, then you can legally drive or keep your vehicle on the road whilst displaying the tax disc that has run out, for up to five days. (Giving time for your new tax disc to arrive by 1st class post).
Hope this clears things up and I will be keeping this card in a safe place till my new disc turns up