Got it sussed. There is a sneaky KeyPresses setting on the main form that needed enabling.
Private Sub mouseCapture_KeyDown(ByVal sender As Object, ByVal e As KeyEventArgs) Handles Me.KeyDown
If e.KeyCode = Keys.Space Then
moutput = 1
m_serialmon.Text = "Serial On"
m_sendingserial.BackColor = Color.Green
End If
End Sub
Private Sub mouseCapture_KeyUp(ByVal sender As Object, ByVal e As KeyEventArgs) Handles Me.KeyUp
If e.KeyCode = Keys.Space Then
moutput = 0
m_serialmon.Text = "Serial Off"
m_sendingserial.BackColor = Color.Firebrick
End If
End Sub