Just to answer my own question: the issues I had in connecting with any phone must have been some kind of fluke. A repair of permissions didn't help, but a restart did.
Connecting to the M600i over USB still doesn't work, but the 3G bluetooth connection I managed pulled more than 100 kbs, which is pretty near the 3G max. For completeness' sake, and anyone else who uses t-mobile in the UK:
Use Ross Barkman's sony ericsson 3G scripts and included instructions:
http://www.taniwha.org.uk/files/SonyEricsson3G-2005-01.sit
From them, Sony Ericsson 3G CID 3 worked for me, lower numbers didn't
In the 'bluetooth modem' tab, Uncheck 'Enable error correction ...' and 'Wait for dial tone...'
In the PPP tab, don't worry if the requisite four character password ("pass") gets multiplied to 23 asterisks once you hit 'apply now' This seems to be normal. Ditto the ten asterisks (or balls) in the internet connect window. The telephone number is indeed 'general.t-mobile.uk' (not a number, nor 'general.t-mobile.co.uk'). Account name is 'user'. Curiously enough, in the 'PPP options' pop-up, enabling 'Send PPP echo packets' didn't make a difference for me. YMMV