So as I understood the process - to draw an arrow with the head starting 3/4 the length and +/-5 pixels wide. It looks a lot better than my trying to work out the angles and hoping I was using the trig functions correctly.

fl_push_matrix();

// Set the drawing to be centred in the widget and rotated by the wind direction

fl_translate(x_zero, y_zero);

fl_rotate(-(double)dirn);

fl_begin_complex_polygon();

// Draw an arrow down from North

fl_vertex(1, -radius);

fl_vertex(1, radius/2);

fl_vertex(5, radius/2);

fl_vertex(0, radius);

fl_vertex(-5, radius/2);

fl_vertex(-1, radius/2);

fl_vertex(-1, -radius);

//

fl_end_complex_polygon();

fl_pop_matrix();