Chris - the T Mobile Compact III is the same phone as the O2 XDA Orbit.
They are both made by HTC, and rebadged (with annoyingly confusing names!) by O2 and T Mobile.
The only difference between them (apart from the colour) is that the XDA orbit can connect to the internet over a 802.11b wifi connection as well as GPRS, whereas the Compact III can only connect over GPRS.
In case I'm just being confusing with abbreviations - GPRS is when the phone connects to the internet by basically ringing up T-Mobile or Orange, like a PC using a modem, whereas wifi is when the phone connects to the internet by joining a nearby wireless network.
GPRS is available pretty much everywhere, but is not very fast, whereas wifi is much faster, but not really available outside central london (unless you've got a wireless network at your house!)
Somehow I get the feeling i've just confused matters more...