I have Phygellius [cape fuchsia] in mid shade under a beech tree and it does really well. You can also grow aconitum, digitallis too.

Thanks Seaburngirl! The garden's full of digitalis, I pull it up whenever there's something better to grow, or when it's swamping the geraniums & aquilegias again. I find Phygelius doesn't thrive in so much shade - I have it in sunnier bits of the garden but it seems to struggle under trees. Any bushes / shrubs to suggest as it's part of a large garden and not easy to scramble up the bank? I forgot to add to the list of "already gots" - Viburnum tinus is doing well.

That's a fair area to fill Beattie. I have various Hostas and Heucheras which seem to enjoy the shade and an Aucuba Japonica Crotonifolia (variegated spotted Laurel) - Oh and a Skimmia which is also in the shade. Hypericum will also spread well and the flowers are so bright and sunny.

Aucuba variegata or A.Salicifolia for yellow spotted leaves and berries 7'x7'. Peony rose mentioned A Crotonifolia I think that is a male so no fberries.
Chaenomeles, orange, red, white, pink or cream flowered available 4' - 6' high
Cotoneaster dammeri is a brilliant evergreen ground cover for slopes & banks white flowers & red berries.
Deutzia will grow in partial shade. Several versions with pink or white single or double flowers. 4'-6' high bush.
Escallonia, white,pink or red flowers.6' high.
Fatsia growsto 10'. flowers late in autumn.
Fuchsia
Hippophae, Sea Buckthorn.
Hypericum Hidcote 6' flowersyellow July to October.

Mahonia aquifolium, brilliant in shade, as well as Prunus 'Otto Luyken'

Thanks for the suggestions. I bought a Deutzia for £2 today in Morrison's so if it doesn't do well I won't have lost much, but it looked like a nicely grown plant. We have lots of Escallonia already - and one Fatsia japonica. I could add another one or two. Got some hardy Fuchsias in B&Q as well (20% off). I thought some more Cornus species might be good too. Eg Cornus kousa, C. canadensis. I'll keep looking out for good ones for this situation.

Good buys, just remember to mix the evergreens with the deciduous and plant some bulbs for spring interest and it will soon look good.