I'm Vodafone UK too, only had the phone for 5 days.

After installing Personal Dataplan Widget today I did a test where I left 3G on and the phone otherwise idle, for 3 hours. It averaged about 42KB an hour, which works out to a little over 31MB a month, if connected 24 hours a day (not likely).
I have Skype and Google Talk maintaining their respective statuses, and the Foreca Weather Widget (not on the main desktop pane). Email is set to update over WLAN only, so didn't have an effect - I'll test that tomorrow.