MMS settings for M3100

[Guest kevlar]

I've just discovered after owning the phone for a week that I can't send MMS for some reason. I rang O expecting to sort it out with a Sim update but was told this wasn't the case and I need to ring them back on another phone and go through the settings, I was down the pub at the time so this was a no go. Can anyone think of what could possibly be wrong in my settings? Anyone had problems with mms since upgrading?