welcome Richardvye,new to GOY myself but you have done what I did posted twice...usualy gets there even if you THINK it hasnt.Its very adictive this site I cant keep off it,but will leave your question to the more experienced gardeners,some good ones on here.

Are the trees evergreen coniferous ones, or deciduous broadleaved? Any idea what they are?

I have a winter sweet under a beech tree and also some bamboos. Also an acuba 4ft from a beech hedge. but yes it would help you if we know the trees and how close they are to where you want to plant.

Mahonia aquifolium often grows well under trees. You might also try Christmas box and butchers broom. Sorry I can't remember the Latin names. I've grown euonymus fortunii under a big tree - it survived but the variegation was poor. If it really turns out to be too dry for shrubs at all euphorbia robbii seems to grow where nothing else will, but its only about a foot high. Spreads rapidly when established.