Hi Tony. Are you sure they are white fly. Beech usually get a downy aphid. If you shake a branch you'll get a haze of powdery dust arising from it. Kinda gets in your eyes and nose if you are clipping a beech hedge. I really don't know what you could use for either. If the trees are managable you could try hosing the undersides of the leaves and trying a systematic spray that will travel throughout the tree. Apart from that just keep trees well watered and fed to withstand the onslaught. I am not certain, but in my experience that downy aphid doesnt really spread onto many other plants apart from apples and currants. If it's whitefly as you know it's a different story.
thanks for your reply, yes I am reasonably sure they are whitefly and nothing comes off when shaken and there are signs of the 'sticky hoeydew' on the leaves, I have to admit this tree is on the fell or not to fell list ansd I am more concerned that it will be a large source of infestation for the rest of the garden inc my veg. the spread of the tree is actually over part of my runner bed and adjacent tomarto and salad crops - it also hides a rather nice view of Dartmoor as well!
I was hoping to wait untill after leafall if felling as it would be easier, however I think it's days might be numbered