Dated 23 July 2010:
Thank you for your response.
The devices I have personally used to confirm that data modification is taking place are an iPhone and iPad. However, I have received confirmation from other users that that modifications are taking place for mobile broadband users using laptops as well, and I know that many of the practices are also occurring for users with other mobile phone devices.
However, the broader and arguably more serious issue is the fact that O2 has a practice of modifying any data passing through their network between senders and receivers without notifying either party of the changes that are being made. Even when I tried to find out what was happening I was told by your technical department that they *did* compress data but that they were not legally required to give any detail of what they did. (I should add that this statement was misleading as they do a lot more than just compress data, they also change and remove data). In other words, even if I ask, I am not told what O2 are doing to my data.
I believe that this practice may nullify the contractual obligations of every customer of O2's mobile data services at the present time.
Finally, I refer you to European legislation being formed around 'net neutrality' <http://europa.eu/rapid/pressReleasesAction.do?reference=SPEECH/10/153> and curently being considered by Ofcom. The Vice President of the European Commission said: "Transparency is non-negotiable. This is already addressed in the new regulatory framework, but the principle is worth re-stating: in a complex system like the internet, it must be crystal clear what the practices of operators controlling the network mean for all users, including consumers."