Move cursor and left click

When a button is pressed, I want the mouse to move to a predetermined location on the screen, and left click.

I have tried:

    Public Const MOUSEEVENTF_LEFTDOWN = &H2
    Public Const MOUSEEVENTF_LEFTUP = &H4

mouse_event(MOUSEEVENTF_LEFTDOWN, 0, 0, 0, 0)
Mouse_Event(MOUSEEVENTF_LEFTUP, 0, 0, 0, 0)

...for the mouse click, but the mouse moves immediately to the bottom right hand corner of the screen, (the very bottom-right pixel), instead of left-clicking at the current cursor location.

The moving of the cursor is working fine on it's own.

Any help much appreciated!