Thanks Eleni and Filippos. I agree KEP would probably be a good starting point but they asked me to put a query on LiC to see if anybody had direct experience.
The land has full planning permission for a house that was completed some 8 years ago so they don't want to remove trees in order to build. The trees, which form part of the garden and front onto a minor dead end road, have become badly overgrown so their plan is to thin out the area by removing about 5 trees, some of which are offshoots of larger trees, leaving behind some 6 or 7 trees that have grown very tall. The density of the trees at present stops the growth of plants and shrubs at ground level.
The trees on an adjacent plot were removed about 4 years ago, legally or otherwise? They do not want to use the land for any other purpose, other than to plant some shrubs around the trees, and they don't want to remove all the trees just open the area up a bit.