The thing you haven't mentioned is whether or not you recently replanted the window box - if you did, its probably squirrels. They do this in autumn particularly, looking for bulbs, or burying chestnuts and the like, especially in newly planted pots and boxes. Its likely the damage is occurring early in the morning, just after dawn - if you fancy getting up that early one day, you might catch the culprit. If it is squirrel damage, its quite difficult to keep them off - chili powder sprinkled in and around the box might help.

Magpies and crows do the same whilst burying surplus food.

I'm thinking squirrels; I caught them "red handed." They are looking for a place to bury their cache of acorns for winter storage.

Thank you, everyone, for your advice. I can now tell you that I have discovered who the culprit is. Very early this morning I was awoken by a scrabbling sound outside my window. Leaping out of bed and flinging open the shutters I saw sitting in the window box ... a fox! We stared at each other for a few seconds, both frozen in shock, then it was off. This morning I found the usual uprooted plants and soil all over the ground below the window box. And some muddy fox footprints going across the garden and over the wall. Now we know it is a fox - can anyone think of a way I can stop it destroying my window box!?

Well, I've never heard of a fox doing this before! I'd still try plenty of chili powder or even ground pepper - if nothing else, the fox sneezing will wake you up so you can chase him off...