Thanks for asking, Fab.
She went back to the vet yesterday when she started to grunt when breathing, but by the time we'd got there, she'd stopped grunting and her lungs were clear, so vet said perhaps she had a bit of food stuck.
Back again today for the planned check up with dental expert, who said that if she is eating greens she's fine, and he wouldn't put her under anaesthetic again without good cause.
Things I have learned: guinea pigs hold food in their mouths for up to 48 hours after eating, which makes it difficult to a) inspect their mouths and b) anaesthetize them. Also that while dental problems stop them from eating, the dental problems can be caused by them not eating, so the underlying cause might be a stomach problem. As our girl has a good appetite, that doesn't seem to be the case.
If she stops eating again, I won't syringe feed her until the vet has seen her, to keep her mouth clear.
The vet told me to put her out on the grass in the sun, so that's where she is now, looking a bit nonplussed, with wet paws. Have put a box of hay in the run for her in case she wants to keep her feet dry.