If you want privacy from the windows at the bottom of the garden, then you have 2 options. The first is to plant a tree down there but this would take a while cover the windows. I would suggest something that doesn't grow too large such as Betula utilis var. jaquemontii. This has pure white bark and doesn't cast too much shade. The second option is to screen off a sitting out area with a closer screen. This could be trellis with climbers on it. A closer screen can be shorter than a far away screen and casts less shade. You could do both and remove the closer screen when the other one has grown. Please bear in mind the eventual height of the tree that you choose and don't go for a very fast growing one as it will grow, and grow and not stop!!

For a privacy screen - Hardy bamboo (faresia) are fast growing, evergreen and make a lovely sound. The will like improved clay soil and south facing and grow to over 3 metres if you want (a pair of scissors will keep them small)

Hornbeam is quite fast at establishing and is easily grown into a high hedge or even a cordon and a summer prune will make it keep its leaves, just like Beech, so it is a screen in winter.

My first thought was a tree too such as Prunus cerasifera 'Nigra' - Purple Leaved Flowering Plum which would give excellent summer screening. Or an evergreen laurel hedge such as Prunus laurocerasus Genolia which has an has an erect and narrow growth, which is ideal for hedging as it creates a good shape, the reason I suggest this is it needs little trimming, which might be a consideration with difficult neighbours?

Thank you,, off to do some research and yes Drc pruning would be an issue as they have already decided they dont like the spirea's i've planted and cut them back before they even started to grow properly.

If the spireas are on your property they have no right to touch them They can take off any shoots growing on to their property.

Hawthorn, Pyracantha and Berberis are all prickly plants and the neighbours would think twice about pruning them on the sly. I had an old hawthorn hedge at my old house and it is a great screen even in winter if planted thick enough.

Portuguese laurel, Prunus lusitanica would be my choice. Evergreen, vigourous, pointed leaves with purple stems. Has the advantage over the cherry laurel in that it looks good even when cut with a mechanical hedge trimmer (having a smaller leaf).

Is prunus lusitanica fully frost hardy in the UK and does it always retain its leaves in a hard winter ?