So- the plan should be to use as much data over the next couple of weeks as you can, (using vodafone internet and not wifi obviously). That way you can show that there will be an extra charge of at least £5 on your bill if the charge is introduced.
If that £5 charge is at least 10% of your ordinary bill ( i.e if you normally pay less than £50 per month for your tariff) then this change to the contract, by vodafone, means that you are affected by more than 10%. This SHOULD mean you can then cancel your contract under the clause mentioned above.

My wife and I are both going to try this and see what happens. My wife wants a new phone ( fed up with her HTC magic) and I wouldn't mind getting a nice, cheap, sim only deal to use on my n900 until it's time that a reasonable successor to this great device appears.

P.S. If I am wrong on any of my assumptions then someone please correct me