How about trying some Hydrangeas? I see that. Focus have a collection of shade loving Hydrangeas on offer. In my last heavy clay garden the back faced North. My favourite blooms were the umbellifer Selinum wallichianum, Ammi majus, Japanese anemones, some of the clematis like shade, Gillenia trifoliatum, and of course snowdrops in spring and the evergreen Euphorbias.

Pachysandra variegata is an evergreen very shade tolerant plant with white flowers, happy in clay too about 6-8" tall generally.

I have martagon lilies [3ft] in quite deep shade and they do well for me as does Digitalis lutea, a pale primrose coloured flower, 3ft is tall flowers late spring.

The bugles may work for you too, Ajuga to give it it's scientific name, blue flowers mid spring.

Some of the silver leaved dead nettles [Lamium] cope under my beech tree.

Have you tried epimedium?

I would look at improving the soil to give a better range of subjects. Having said, I would go for the lower growing Astilbes. These will cope with the heavy soil, shaded situation and flower a little earlier.

Thank you for asking this question, Pennyfarthing, I have this problem too with a flinty clay, small patch bounded by the brick wall of the house on the East and South and crazy paving to the West and North. I masked the central heating vent with a 'dwarf' conifer but it wasn't happy in the rain shadow of the house, especially in the recent hot Summers.I have a pyracantha trained on the wall to the East and orange flowered montbretia growing below it, ivy ( wish it was variegated but it is neat, small leaved and softens the hard lines) on the wall to the South with Elephants' Ears growing in front of that, with lily of the valley, bugle and pulmonaria under a dainty fuchsia edging the crazy paving on the North side. Over the years across the bed I have planted out my forced hyacinths in late Spring and the Winter Rose a friend gave me last year is spreading there too, between the alchemilla mollis which happily multiplies each year. I had bleeding hearts growing there too until the temperatures plummeted last Winter. It sounds quite full but every year weeds carpet the bare patches of soil and the intended floral colour range is quite limited each season, a splash of red or yellow would bring it to life. I transplanted a yellow cinquefoil into the bed a couple of years ago but I am not sure it is all that happy. I toyed with turning it into a fairy garden dotted with miniature solar powered residences but I don't think that it gets enough light to make that viable.... shame because it is the focal point in the garden viewed from the gates at the end of the drive.

I love browsing https://www.plantsforshade.co.uk/