Answer
Its worth upgrading to the highest tariff you can afford.i had a similar situation with mines though it was more with internet access. my phones free allowance was 2mb, and i went over this to the tune of over £20.however, i phoned o2 who have bolt ons which give you 200mb a month for £7.50. if this is your network then phone and ask if they have a texting one. if not, give your network a call anyway, they might be able to help somehow.mrsc - yes they can, they do allow you to change your minutes and texts, thats exactly what i done when i saw my tariff, i really didnt want a heap of texts and got more minutes instead.
