Been searching these for ages and you wouldn't believe it! I found working settings here on this forum!! lol Thanks to Jleagle.
Follow this and type in the settings and your MMS should be working as mine works great now!
http://androidforums.com/nexus-one/37742-faq-getting-your-internet-work-o2-mainly.html
Read further on down though as there is advice on using 'Handscent.app' to resize images etc so that they can reduced in file size for sending using O2.
Remember that I am also using an ex-orange phone on a simplicity contract.
Goodluck!