Switch to using inset box-shadow

packages/components/header/_header.scss

@@ -534,11 +534,16 @@
   }
 
   // Current items within the expended mobile menu get
-  // a blue border to their left, and some padding to make
+  // a blue line to their left, and some padding to make
   // them more visually indented.
+  //
+  // The blue line uses an inset box-shadow instead of a
+  // border, as there’s already a grey border on the top,
+  // and using 2 different colour borders means an awkward
+  // diagonal line where they meet.
   .nhsuk-header__navigation-item--current {
-    border-left: nhsuk-spacing(2) solid $color_nhsuk-blue;
-    padding-left: nhsuk-spacing(1);
+    box-shadow: inset nhsuk-spacing(2) 0 $color_nhsuk-blue;
+    padding-left: nhsuk-spacing(2);
   }
 }