What I do admit is that variation clauses are frequently used and lay down the requirements for this to take place. Insertion of such a clause or any other clause/Term does not automatically make it legal and yet to challenge the validity of it would require either mediation, regulatory body review or in court. This is the variation clause in PN's Ts&Cs:
In reply to:
24. How this Agreement can be changed
We may change this Agreement, including our Charges, at any time. We will give you at least 14 days notice of any changes before they take effect. You may not change any term of this Agreement. As explained in clause 23, you can end this Agreement by giving us immediate notice in this 14 day period if we increase our Charges or change the Conditions of this Agreement to your detriment.
The reason why they are allowed to do this is because the customer can simply migrate away to another provider and because PN won't hold them to any remainder of the contract term. PN have not given notice of change of contract that should IMO have occurred with the introduction of LLU and nor did they do so when they reduced the limits across the board in April 06. The reason for the latter was claimed on the basis that the SUP and AUP were not part of the Ts&Cs, then it was part of it but not detrimental and finally that the hours for peak time were reduced hence no detriment. It is the PN way.
However and what would be interesting is if someone refused the changes and demanded PN provide the contracted for service for the remainder of the duration.