Sheltered shady corner with heavy soil

I'd really like to fill this area of bed better but struggling to find things that thrive there. The soil is heavy compared to the rest of the garden, though better than it was thanks to repeated mulching over the last couple of years (what you can see in the picture is the latest layer of that). It gets little to no direct sun even in mid-summer. The only plus point I can offer is it is incredibly sheltered (the wall behind is 10-12ft high)!

I'd like to add a little height/interest to the area; it's next to a small patio with a bench on, which is a nice sheltered spot to sit but just feels a bit bare and empty at the moment.

 

Hiya 2nd_bassoon
Sarcoccoca would grow there.....evergreen, easy and very fragrant (here they are now filling the garden with scent). They will grow to 6' plus if left unpruned.
Hackonechloa from spring to autumn is beautiful ......for me the most classy grass.....as is the japanese painted fern (Ursulas Red is a good variety with blue/grey leaves, red veins and red stems).
The dogwoods will provide an evergreen theme there.....red stems when the leaves have dropped. Great backdrop to the yellow hackonechloa and grey fern.
I would also plant a hellebore or two there.....a single or double white flowered variety would light up the area. Ophiopogon nigrescens with its black leaves will look great in front of the white hellebore
Skimmias too....very under rated shrubs but go for a hermaphrodite form (the cornish bred Redruth for example) will produce berries on its own.
Of course hydrangeas. The paniculatas will flower on current season's wood ....Limelight here is a delight) but if you prefer a small variety check out Bombshell; a cute little neat shrub with white flowers all summer long. Again, the white flowers stand out in shade.
Liriopes are evergreen grass like plants with purple flowers in late summer into autumn...nice.
Check out the tiarellas......similar to heucheras but with better flowers and better in shade.
Hostas too, an obvious choice......for such a sheltered sun limited spot I would go for Fire and Ice. Almost white leaves. Stunning
A thick organic mulch or, to match your stonework, a gravel mulch would finish it off nicely I think

 

Many many thanks @Verdun! Lots to think about there.

Hellebores are already on the list. I'm a bit hosta obsessed but they don't do well in the ground for me here. I love the tiarellas, and, surprisingly, the hydrangea - normally not a fan of those. Like the idea of a bit of scent from the sweet box too. And grass-type plants hadn't occured to me; I love grasses but don't have space in the sun to do them justice, so this might be a good compromise

 

Pleasure 2nd bassoon.....you can make that a very attractive and perfumed spot I think