I dont´t know the name in english, but in latin: Ulmus pumila celer...unfortunately not from my own experience, but from friends´s - growing 1,2-1,8 meters /year....

Better to plant different trees as you may run into trouble if its later judged to be a hedge when its over 6 feet and could in theory be told to reduce /remove it.

Betula pendula (weeping silver birch) is both quick and slender, but might not be thick enough to use as screening, plus it's deciduous.

Good Idea Sid I have some to one side of my garden they thicken up, but grow very high 15-25m. Also with any trees you have to consider root spread if near buildings?

Good point Drc. As far as I'm aware, B pendula has a fairly fibrous root system.

'Spartan', 'Hillspire', or 'Spearmint' Junipers. Maybe 'Wil Fleming' Yaupon Holly, if it's hardy enough.

Thank you all , your suggestions will be passed on.