Thanks for all the replies It's great to whinge to someone other than my partner
As I said, this was very much a vent... I'm not sure there's a LOT we can do. It's so tempting to cut her off, if only briefly to have her appreciate what we do. Unfortunately I don't think it would help - she either honestly thinks that it's no different to doing nothing, or she's too stubborn to admit that we're of some help to her.
I like the suggestion of clear boundaries though - particularly with regard to specific times/days for help with shopping etc. Will definitely suggest that to my partner. Maybe ask her to come up with the things that would be most useful to her, and we can agree in advance on what we'll help with (unless it's an emergency). Hopefully that'll help.