I suspect I know the answer to this, but...
Since the 306 passed it's retest after I did the brakes, I've been and bought a shiny new 6 month tax disc. I didn't bother with 12 as I reckon it'll be unused again for a couple of months come the end of the summer.
It is valid until the end of September. Question is, can I use it from today, or is that a bit naughty?