I have the same 'problem' and have found it hard to get much that will flower in summer as the shade is very dense and the ground is dry (even here!) so I have looked more to foliage interest.
Aucuba japonica and laurel both grow happily at the back, as well as a Skimmia and a small gold and green Holly. There is also a camellia, but I am going to move that, as it never gets enough water to flower. My little Rhodie yakushimanum has flowered a couple of times, but that is getting moved too, as the lack of bloom there is mainly down to excessive pruning by sheep! As it is near the yard gateway, it is the first nosh stop on the way in for invaders!
I have Pieris 'Little Heath' in a pot in full shade and it is quite happy, so that or another Pieris might work there too
As ground cover I have Lamium galeobdolon, a bit of a thug but not too hard to control, and the silvery leaves and yellow flowers are nice. I have Galium odoratum elsewhere that I am going to add, as it should do well, as wouild Vinca - the variegated leaf V. minor would be nice and lighten things up too.
Ground elder grows there, of course, so you could try the variegated form of that too, if you dare! It is reputedly less invasive than the green version, but that is a relative judgement
Last edited: 05 August 2017 08:58:31