Oh dear at the fizzy mango - I've made that mistake with fizzy pineapple before!
I still attend the NCT coffee mornings and I've made a great circle of friends there, I'm also very active with our branch - I am their newsletter editor.
I think with all groups, whether they be NCT, toddler groups, bf cafe etc etc, you sometimes need to be quite tenacious and put yourself out there. People will often see a new face and bring you into the conversation, but sometimes you also have to sort of push yourself into it. Some groups are well established and can seem quite cliquey, but that is because they are have been friends for a while so have lots to talk about, but don't be put off, bring yourself into the conversation and start conversations off. Even if you get onto a subject you're not that interested in, keep chatting and stay in the conversation.
I know that all seems rather scary and daunting, but I am quite outgoing and I still found I had to bring myself into the group - probably because they were all well established groups. If it all sounds a bit too much then I suggest you go with a friend, even better if your friend can talk the hind legs off of a donkey!
Groups really can be a lifeline. There will come a point when you've had enough of spending day after day at home and you feel confidant enough to start going out and about with your baby and meeting other mums.