Round and about in Epsom (1)

I've heard of and occasionally come across pleached limes (see, for example 12th August), but I have not come across pleached leylandii before - or, at least, that is rather what they look like. Captured out the front of Bourne Hall Library.

And if they really are leylandii - and not, say, some kind of yew - what on earth is going to happen as time goes on? Someone has clearly gone to a fair bit of trouble and expense so I must take a closer look and get proper identification off the thing