Like have a 90ft south facing garden which has loads of trees in it. I also back onto a woodland with a lot of big trees. Therefore the garden is very shady, but everywhere gets some sunlight throughout the day.
The other day I decided to dig up two shrubs that have done nothing sat under the willow tree. What can I plant in their place?
Sounds like your main problem will be dryness? In my garden things that grow well in dry shade are mainly spring plants/bulbs and woodlanders. Cyclamen do great under trees. Geranium macrorhyzum will give you summer flowers. Hellebores do well for me too. Some ferns are happy in dry shade too. Pulmonaria will probably be OK but may have floppy periods if it is really dry.
Brunnera could also be worth a go (though I dont have it so can't speak from experience!)