[Paul Stenning]

A series resistor is needed or you will destroy the LED.

R9 on the PCB is the LED series resistor and would be set to suit the LED being used and desired brightness. Around half the LED rated current is a good starting point; for a 2mA LED I would try a 6K8 resistor (assuming 2V for the LED and 7V across the resistor) to give around 1mA as a starting point.

Higher resistor and thus lower LED current is better for battery life, as long as the LED is bright enough to act as a useful indicator.