To quote their own T&C's:
Changing these Terms (notification)
Sometimes, we will need to change the charges and the Terms and Conditions of the service. We will publish details of all changes online at http://www.plus.net.
We will also let you know about a price increase or a change to the Terms and Conditions that we believe is likely to cause you material disadvantage at least one month before it happens. We will let you know about other price changes and changes to the Terms and Conditions via email to the contact email address on the account and by adding a service note to your account. Account Service Notes can be viewed at http://contactus.plus.net/
For changes we need to make to meet legal and regulatory requirements, we may not be able to meet the timescale detailed in paragraph 44. We will let you know about these changes as soon as we can.
If we have made a change which is to your material disadvantage, you will not have to pay a charge (excludes any charges or payments we have deferred) if you decide to end your agreement early, unless the price guide says otherwise. However, once we have told you about such a change, you must let us know that you want to end the agreement within 10 days.
The sentence I'm interested in is We will let you know about other price changes and changes to the Terms and Conditions via email to the contact email address on the account and by adding a service note to your account.
I agree this can be read in several ways, however to me that would imply that any changes to the price, up or down, will be communicated out by email as the previous sentence specifically states that if there is a price increase or a change to T&C's then this would be communicated out at least one month prior, so the following sentence should be a catch all for anything not related to a price increase or change in T&C's.
Now, they HAVEN'T contacted me or anyone else about this price decrease to our service because they clearly want to keep the money for themselves so you could argue they didn't have to tell me about a price decrease because they didn't change MY account, however they have changed the SERVICE I get to a lower price and surely that constitutes an email about a price change?
Again, it's not the amount, it's the principal
plusnet 4/11/2003 -> 4/3/2009 512Kb-2Mb
O2 Standard LLU 4/3/2009 -> 23/6/2009 ~3Mb O2 Access 13/7/2009 -> 22/6/2010 ~8Mb
plusnet 22/6/2010 -> ?? ~8MB