Apparently if you send a message to more than one contact, they are automatically sent as group MMS messages and it doesnt matter even if you use just plain text and under 160 charracters. I checked my account online and yes I am being charged for 11 MMS messages that should have been plain text messages. Thank god my Dad told me and I spotted this early.
I have sent a message to O2 Ireland and logged the issue on Apples Developer bug reporter website but I am interested if this is effecting others when sending texts to more than one contact.
For those of you worried about being overcharged you can disable MMS in SETTINGS > MESSAGES > MMS MESSAGING > OFF
iPhone Os 3.0
O2 Ireland Carrier