We now have a theory about why the piggies were nibbled (though not who did it). I mentioned it can be a sign of stress.
Last week we had to remove their little wooden house temporarily on a couple of occasions, because they had got it wet and it had to be cleaned and dried out (guinea pigs don't do well in damp conditions). This definitely freaked them out a bit and they all crammed into the other shelter, which they don't normally sleep in. I wonder if that's when the nibbling happened.
In any case, we have made a trip to the petshop and bought them a new plastic house, which will be readily washable and should give them a safe place to hide even if we have to take their wooden house out again. You're really supposed to provide one hiding-place per pig, and while we had the wooden house, a cardboard tube, and a shelter, we didn't have a spare if any needed to be removed.
No more nibbling has taken place. Fingers crossed.
We also went to IKEA yesterday and bought various household items, including a salad spinner. Apparently my husband has always yearned for one. Known him for eleven years, but he can still surprise me!
So we are not going to worry too much, and going out for Sunday lunch. See you later!