I'm not sure if this is good advice but it has worked for our pigs and bunnies, when they have been off their food. Its purely a natural thing, so surely it wouldnt harm. All of our pets love grass and dandy leaves, but obviously in this weather its hard to give them any (never feed them cold/icy food) and they will have reduced goodness because of the weather - but I collected a good handful of clean, healthy looking grass and some nice dandy leaves, and gave them a very good long wash, chopped it all up into tiny pieces with scissors, washed it again and let it soak (and importantly - warm up) for a bit. Then after drying the mush for a bit, I add a bit more clean water to make it fairly wet but not too wet - just enough to increase their fluid content.
Its kind of cheating, because they obviously adore grass/dandys, so are always sure to nibble away - so giving some of the pigs and both rabbits this in an easy to eat and hydrated form seemed to work wonders and kick started their appetite on each occasion.
I guess its another version of Science Recovery, but it seemed to do the trick.
Good luck at the vets.