Guess you've tried ferns and pulmonarias? I have Helleborus foetidus and Iris foetidus and one or two other Hellebores in deep shade and they still flower. Brunnera Jack Frost? Guess you've thought of all these though... and how about a piece of garden sculpture or some some other sort of ornament as a focal point? Is it too dark for a variegated Euonymus to keep its variegation?
Ladies Mantle might do too.

Pachysandra terminalis will as will a range of Disporum, Podophylum , Polygonatum, some of the Epimedium etc.

In addition to the list above I also have Dicentra, Ajuga, Muckdenia, Mianathemum [3 different species] in a similar area in my garden. Though yours looks much bigger. There is always some nice large leaved ivies to consider if you want to try them. I admire them in other peoples gardens but are reluctant to try them in my own.

Looks ideal for hydrangea esp my white one. Look for 'Strawberries & Cream' and 'Incrediball hydrangea'. Give them a Google. A hedge of Icrediball would be amazing!

Ta. Bit too dry for Hydrangea. Find that they do not do well here.
Got all the others mentioned so I can at least try them without spending huge amounts. Horrible feeling it will get no water at all as the tree coverage is very thick. The hedge behind also sucks out a lot of the moisture.

Maybe consider one of the viburnums - Mapleleaf, Snowball, Doublefile, Cardinal Candy, Shasta, Leatherleaf (I have this one) are very popular with attractive foliage, flowers followed up with berry clusters in late Summer.

Owd the last house we lived in had a border so dry that even hardy a geranium drooped. So I dug it up and put some moisture retaining granules in the planting hole. It worked and the geranium was fine after that. I do that a lot now if I think a border will remain on the dry side.

I'd also suggest mahonia nervosa but can't locate a supplier at present

Mahonia aquifolium should do well there...

Thanks.
Stera, just ordered some Water retaining gel. Never thought of doing that in the garden soil.
That section of the garden gets no rain on it from when the Beech, Oak and Birch get their full leaf cover.
The Euonymus along the lane are in just as deep shade and they keep their variegation, so may try them as they are cheap and available.

I have several Epimedium in dry shade, and Sweet Woodruff (can't remember its Latin name) scrambles around freely for good ground cover.