Kev, the brake is adjusted by turning the nut which you can see on the very end of the cable where it goes through the brake arm at the front wheel, You will probably need pliers or a spanner on the nut to turn it.
Turn the nut and you will see it clicks on a cam system each full turn (you will feel it climb over the cam each turn) make enough full turns until the brake lever feels right for you, this depends on the size of your hands/fingers, but you should set it so that you can pull it pretty hard without it touching the handlebars, the same as on a bicycle.
Make sure the nut is turned a full turn each time and not left sitting up on its cam midway through a turn, you will see what I mean when you do it.
When you have finished give the wheel a spin by hand and make sure it is turning freely, as if you adjust the brake too tight it will bind and the wheel will feel tight to turn.
Hope this helps.