The petals on all our snowdrops in just one part of the garden have all disappeared and the petals on some Tete-a-tete daffodils are going the same way.

I don't think it's slugs or snails because I have been out several times in recent mild, damp nights with a torch and there were no molluscs to be seen. The bed is, however, alongside a very old dry stone wall which is riddled with mice holes at the bottom (possibly also bank voles). Would they eat the petals of flowers as a delicacy?

Morning Monika, i would say there is a very good chance of it being mice, especially if it is happening at night.

Slugs probably. My daffodils were badly attacked until I put out slug traps. Now they bloom undamaged. They love stone walls and can hide easily. I was unsure until the traps collected dozens of them.