*MOAN* Fonehouse

[Kitten]

I would change the first bit of the letter and just simply say that the reason you're not paying it is not that you can't afford it, but:

I am currently in discussions with T-Mobile regarding this debt as I believe it has been wrongly charged. The reason for non-payment up to this date is, simply, that I do not believe that I owe it and as yet, no-one has been able to prove differently.

then later on:

I will not pay the £100.71 until this issue is resolved with T-Mobile and a mutual agreement has been concluded. If this is found to be an error at T-Mobile or Fonehouse systems, please be advised that I will be writing to request payment of the time I have spent trying to resolve this issue at my hourly rate of 'xx' and the cost of the telephone calls made. I can provide evidence of this time spent and shall do so, if you so require.

There's very little chance you'll get anything back but you've got every right to do it and they know it.
Sorry, got to go to work but will try & check on this later.