Hosta. There are thousands to choose from.

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ferns will love these conditions too and there are many that keep their foliage all winter, and then you just cut them off in the spring when the new fronds emerge.

some of the skimmia will do ok as will acuba but they will both get large in time but they can be pruned. Holly again not small but responds to pruning. Euonymous might manage. I have a green and white one that is under the beech tree. what about wintersweet [scientific name on the tip of my tongue] again I have this in ddep shade. evergreen daphne also in the shady part of my garden.

There's also sarcococcus which is an evergreen. I have a similar area and have two ferns and a pyracantha there. All seem happy.

There are over 3000 varieties registered to the American Hosta Society. Just wanted to make sure that what I said was not an exaggeration. They also love damp boggy conditions.

Damp, shady places are also loved by slugs, and even more so when Hostas are on the menu! And after the mild and very wet winter we've had in the UK, the slug population has increased dramatically.

As such, I would avoid Hostas, and plant evergreen, shade-loving ferns. Make sure you pick those that will happily grow in your particular soil, though. For example, Blechnum Spicant will only succeed in acidic soil, so do your homework first. :)

Hostas get my vote any day!

Dig in some gravel and use hellebores as companion planting to have interest through most of the year.

How small is "small"?

Thanks to all who gave me some ideas.
"Small" is about 60 cm x 180 cm. As it's outside a window, although a little away from the house, I don't want anything that will grow too tall and take away the light from inside. Hostas seem a good idea (perhaps all the slugs will stay there and leave everything else alone!)
I'll leave a comment when I've decided. ?

That rules out shrubs then, except for the euonymus, which can be clipped to keep the size you want. Evergreen and variegated gold or silver. (need the variety Fortunii)
Very labour saving as when grown it needs practically no upkeep except occasional clipping after several years.

I have never had a slug problem and the reason for this is the many slug predators in my garden, frogs, birds, tiger beetles, snakes, centipedes and whatever I don't get to see rummaging around at night.

Sarcococca was the name I couldn't recall. Thanks Arbuthnot. that is a small shrub and is easily kept in check with pruning.

Sarcococca Confusa? Fabulous scent but I don't know about keeping it small.

I've gone out and bought some hostas which I'll be planting this next week. Thanks again, everyone!☺️

Great! You will find that they come back every year bigger and better than ever.