I've put a post on the O2 forum, I wonder what that'll bring! The customer services closed at 8pm, I'll phone them tomorrow if I still can't send texts.

Ok, I've changed my sim over to my old phone. (I say old, I've had my 'new' one for nearly a year!) and I can send texts from it, all is fine.

I'm beginning to wonder if I've accidentally changed the settings on my new phone. Don't know how I've done that, right enough. And then would that not change the settings on the sim and not the phone? Jeeso.