Hi Simon

If this is the Sevcon MC400 Series, which is the black logic card which sits on top of the controller.

Then for checking open circuit gate, the thyristors usually reads on resistance range checking between one side of the thyristor to the gate lead anything between 10 to 50 ohms, but does vary.

The pot voltage can be checked on the 15 pin connector to the logic card (connector B). On pin 2 you should get 5v rising to 10v as you press the accelerator.

On pin 4 around 5 to 6v and on pin 14 around 10 to 12v.

Also check you have battery voltage on pin 10, usually when start switch is activated.