Use a regulator with an enable pin. The LM317 doesn't have one.
Or put a transistor in front of the LM317 to cut off the 12V.
davivid I think Your wiring is a bit wrong, it should be like this.Thanks Erni, I have now amended my circuit.
Is this better because your not running the high the current through the transistor, and therefore can use a cheaper cooler running component?
I am interested in knowing the reasons for the design.
Unfortunately I am unable to test until Monday, when I can get hold of the additional parts.
Also I am confused about which part sets the the LM317 - I thought it was a resistor 1.8, between ADJ and OUT, but here its quite different!