Changes to the T&C's on 3 Mix & Match - end your contract in a month

http://www.hotukdeals.com/forums/showthread.php?t=113342&highlight=vodafone

I've recently upgraded with the 3 network on a new Mix & Match tariff and received a letter with changes in the T&C's:

'From the 11th December, any call lasting 59 seconds or less will be rounded up to a minute. After the first minute, you'll get charged for each second, and each second only. So there's no need to cut your calls short, just take it easy and get your money's worth. This change won't affect any of the free stuff you get, like free voicemail. But if you've got any questions, the answers are all at three.co.uk/mix&matchbills'

10. Ending this agreement and Disconnection of 3 Services
Pay Monthly Customers
10.1 You may end this agreement in the following ways:

1. In certain circumstances under our Returns Policy. You may need to get in touch with 3 Customer Services to arrange Disconnection.
2. You can end the agreement during your Minimum Term (if you have one this will be stated in your Price Plan) by giving notice to 3 Customer Services at least 30 days before the date you want to end the agreement. However, you must pay us all the Charges you owe, plus any Cancellation Fee for your Price Plan (as set out in the Price Guide).
3. On 30 days' notice, outside the Minimum Term. You can end the agreement if your Price Plan does not contain a Minimum Term, or if you want to end the agreement at the end of your Minimum Term or any time after your Minimum Term has expired, provided you give notice to 3 Customer Services at least 30 days before the date you want to end the agreement. (A Cancellation Fee will not be charged.)
4. Within one month of a detrimental variation to your agreement. You can end the agreement within one month of us telling you about a variation to your agreement (which includes your Price Plan) which is likely to be of detriment to you. You must give written notice to 3 Customer Services within that month and your agreement will finish at the end of that month once we receive your notice. (A Cancellation Fee will not be charged.)

I've already typed my letter and enclosed the text below:

As you can see from my call logs, a lot of my calls (are less than a minute) and my previous price plan only charged per second billing. I find the changes to the terms and conditions from the 11th December totally unacceptable as they WILL have a detrimental affect on my current Mix & Match bundle. I've calculated how the changes will affect my current usage based on my bill dated 25th August 07 (bill enclosed). I made a total of 71 calls under 1 minute, the total call duration of these calls were 23 minutes 48 seconds. When this is compared to the proposed changes that will come into affect on the 11th December I would have used 71 minutes of my Mix & Match bundle. An increase in usage of my allowance by 302%. I've also enclosed my bill dated 25th September 07, I was away on holiday for 3 weeks but the usage is pretty consistent with my previous bill. I made a total of 28 calls under 1 minute, the total call duration of these calls were 9 minutes 35 seconds. If all the calls under 1 minute were rounded up I would have used 28 minutes of my allowance an increase of 299%.

This wasn't even mentioned when I upgraded and I know I'm within my rights to disconnect. This seems pretty similar to the proposed changes by Orange charging for 07978 calls.

Ducky adds: Thanks Jatt for an earlier pointer.