Im with you shire... i've been thinking about this myself.
From what i've gathered, pre-'06 cars will go up from March 2009 to £300, and then to £440 in March 2010.
So, if you buy 12 months now at £220, but then by your next 12 months at end of Feb next year, thats the best possible solution for you.
You'll be wasting 3 months of the existing tax (3x£18=£54), but that'll still be cheaper than paying the full £300 (£220 + £54 wasted = £274 for next years tax) - taking you to Feb 2010 - and then you can take the £300 tax for 2010-2011 as its before March, and not pay the £440 until 2011.
Doing this would save you £26 next year, but then £140 the following year.
Whether or not the dvla alows you to tax your car before the disc expires i dont know... but they must do surely, i.e. if you've just bought a car, or if you declare it sorn... there'll be a way to do it - can anyone confirm?
Im just throwing out ideas here so please check out what i've said before you commit to it And if anyone else can confirm or deny my theory??