Species: Betula utilis.
There are 5 silver birches in the garden, they grow well, and never get too heavy, so they don't block out the light too much in summer. In winter time their white bark is really pretty and gives structure to the garden. They are mainly planted as a privacy screen. They must only be pruned when dormant or the sap can bleed and they may die. I haven't planned to have them pruned, but if you want to reduce them I would advise getting a tree surgeon to do the job.