Update:
All day yesterday I kept an eye on her.
First she was ON the tent, then I found her INSIDE so she was obviously moving though you wouldn't know it to look at her .
Late yesterday afternoon I checked her and she was again on the mesh tent but this time she looked like all her squidgy bits were escaping the shell, some of it seemed to have a gap where the shell is.
I thought that was it then, she's a gonner.
The foot underneath was very deep, deeper than I have ever seen it and I could see the layers of striping but apart from the feelers you can barely see her move. There seemed to be masses of body in a funny shape hanging round the shell.
By bedtime last night (mine - not hers) she had made it to the glass and was just at the edge of the water, and by now she appeared to be mostly back in the shell and more snail like - which she certainly wasn't earlier, very gross!
Since I changed nearly all of it yesterday morning I'm sure it can't be water quality, although it has occurred to me that being stuck in the shell for that length of time there may have been a build up of toxins.
I decided to leave her and see what was what this morning.
Checked again first thing today and she was in the same position, right at the waters edge, this time with a feeler out of it.
I have now changed most of the water, taking it to just over the top of her so that the feeler is covered.
She is still there, with the feeblest of feeler curling but still seems attached to the glass.
What a saga.
My husband started off by saying for goodness sake, it's a snail and it wasn't that expensive, stop fretting.
Not the point though is it.
I thoroughly dislike snails in my garden but this little one was chosen to go in my tank and I have the power of life and death over her and her fishy friends, depending on my ability to look after them.
To just not bother isn't in my nature.
Other half is now showing interest in her well being, or at the least has stopped teasing me about her which is something I suppose.
Another day gone , maybe she'll make it.