I'm an Arch linux user, not Ubuntu. But I think I can help...
There is a new application available to linux now called
Sakis3G which should solve your problem and make connecting very easy. It's well worth looking up, it has support for a large variety of modems and service providers.
I've used it with ZTE and Huawei sticks and with O2, T-Mobile, Three networks and of course with Vodafone (the older 1GB £15 PAYG). It does not have the means to top-up or read emails tho. It is simply a mobile broadband/bluetooth connection manager.
http://www.sakis3g.org/
It's a very small shell-script with few dependancies (utilises pppd and/or wvdial to make a connection) and it contains an embedded Usb-ModeSwitch.