An interesting topic, which I'm sure most car owners have considered this year.
If you have done your sums and it works out to your benefit, then I guess it really comes down to whether you can fit your lifestyle into a smaller car.
If you can, then your current car is an expensive way to transport a big box of fresh air around.
If not, then consider a similar-sized model (laguna or megane tourer), they have engines which will give the low CO2 tax banding benefits, plus as the megane will soon be replaced, you can probably get a good deal. Likewise, sales of larger cars are very slow, so a new laguna is probably not out of the question either.
I have no experience with any of these cars, but its just my observation and input.
Check out car supermarkets and online brokers, as there are mountains of metal out there remaining unsold.