As I suspected really

http://www.t-mobile.co.uk/Dispatcher...plans_webnwalk

Unlimited surfing, which is great, but subject to terms:

To ensure a high quality of service for all our customers, they are not to be used for other activities such as (but not limited to): modem access for computers, internet based video/audio streaming services, peer to peer file sharing, internet based video download and internet based telephony. If such use is detected, notice may be given, after which network protection controls may be applied which will result in a reduced speed of transmission.

No mention, however, of a fair usage policy on actual usage amount.

Great news, really.