Does anyone know anything about gardening?

Yeah. I'm not one for suggesting people do things just because it's what I would do (because I know fuck all and have zero taste). But for leylandii, I see no redeeming features. Whatever purpose they are serving, there are likely many options that will be less of a pain in the dick in 5 to 10 years' time.

There were 5 in the garden at my old flat that had clearly been planted to form some sort of boundary hedge for a shared path that led to the back of neighbouring gardens. But they had not been contained and we're 100' tall, in a garden that was about 15' x 10'. I got those taken down and it made our garden and my neighbours garden way nicer places to be (dropped "needles" and lack of light were big problems). There were another braces of the fuckers in another neighbours garden that I never managed to convince them to get rid. But then I moved. I'll see if I can find the before and after.