DH has offered to go along to the classes for a couple of weeks, because these are unavoidable one on one meetings that I can't avoid without being pointedly rude. DH commented that when he pitched up on his own the last time neither she nor her DH asked where I was or why he'd come instead. He thought that was odd too because he never ever goes. On reflection, if her DH pitches up without her I do ask where she is and if she's OK, assuming she might be ill. Is that intrusive....?
After I do start going again, I might try and take knitting along or sewing so I can pretend to be absorbed in a task if there's silence. And I'll try not to fill the silence. BUT (and here's why its all become very very hard work for me) I am concerned that even that might backfire on me a bit because I suspect she's a bit 
of the fact I sometimes sew and knit for my DCs.
I am also going to try even harder to keep conversation on completely impersonal stuff if I can. That's harder than it sounds when you're monologing (is that a word...?).
Even after reading all the comments, I'm still no clearer really on what to do or how to go forward. But they have all provoked thought. Thank you for taking the time.
I feel cringingly appalled at the possibility I may be talking about myself all the time, or boring her to tears etc. That makes me feel ill. But if I am regularly 'monologing' to near monosyllabic responses then it is possible I WOULD sound totally only about myself and my interests, IYSWIM. I could sit in silence but its not a comfortable silence.
I have no way of knowing if it is me or the friendship that's the issue without asking her. And yes, I should be braver about that. But I am not comfortable with being that direct and telling her I think she's been off with me and not enjoying meet-ups, partly because I am an introvert and I hate that kind of confrontational stuff, as much as I respect it in others.
If it I knew it was me, then this would be easy. (Although how I disengage and tell her to remove me as her emergency contact and turn down her invitations is still all difficult to figure out too).
But I am worried still she's depressed or ill. If that's the case then I need to hang in there because I am her friend (in my mind anyway!) She has referred to meet-ups with me as her 'lifeline'.
I don;t feel I can ask if there's anything up. I feel that is intruding if, after so long, she's said nothing. It's up to her.
My gut is that regardless of whether it is the friendship she has got tired of, or whether she's depressed, both situations amount to her currently using me as a means for her only DC to have someone to play with: my DD. I understand that, but I don't like it if it means I do all the very very hard work. If she's not ill, its borderline contemptuous to not even try and keep up an easy but casual friendship. I'm not a bad dislikable person.
I wish I hadn't seen those bloody looks which I simply cannot ignore without compromising my own dignity. If I hadn't, I'd still be going along assuming there was something up and it would come out eventually. How can I be her lifeline, but also someone she can be snide about? It doesn't compute for me. But I have had enough and need a break. So DH is stepping in, and he has the sensitivity of a charging rhino, so ... fingers crossed and helmet donned.