thanks everyone
Well it was pretty funny in the end. Got the train armed with an expensive (though slightly horrible) wall hanging as a gift.
Then, after about an hour on the train I received a text (SMS for you US people) from F - the mother. It said:
'can we expect to see you at the church? You are not the godmother. Instead it is a mad religious woman. I got into a situation I couldn't get out of'
I had been asked to be the godmother last December, so naturally I was surprised at this! Obviously many people would have been highly insulted at the change of plan, but actually I felt relieved.
Mainly because I don't have a religious faith. (Neither does the godfather or the father. F is the only one who believes.) So I just felt pleased that I wouldn't have to publicly renounce the works of Satan or whatever they do. I quite like Satan

Bear in mind that I am still godmother to her elder child anyway. So I basically wasn't annoyed at all at this sudden demotion in status, but it does seem rather unorthodox to hear about it on the train on the way to the christening LOL.
So I got to the church, was friendly, and then later said to F: "I know we have really different lives, but we should hang out together without the kids sometime because I am still fond of you".
She did actually seem to respond emotionally to that. And then I thought, well now I can be honest about my life and if they give me any grief over it, then no-one can say I haven't tried.
So I explained in a confident, unapologetic way about why I'm doing a load of study in my mid-30s. And for the first time in ages, she seemed to be able to handle it. Or she did a very good impersonation of it.
So maybe you could say the day was a triumph of friendship. I think F can still be a complete nightmare of a person.
I think this is because her own mother abandoned her, and she has put EVERYTHING into the mothering role as a result. Therefore she can tend to judge others on whether they are a good mother or not - that's her gauge for all human beings, animals, rocks etc.
But if she wants to keep me as a godmother to her daughter then she's gonna have to be nice to me.
On a hilarious final note, I hear she's already fallen out with the new godmother, as they have been bickering competitively about their children. That's why F referred to her as "a mad religious woman".
So that was my ridiculous Sunday...
Here's to a new era in trying to be as honest as possible with people about who I am and who I'm not!