Was advised by the shop to lift the plastic cover on the ESC to make sure that the set button is being pressed down properly. I have tried this. Still the same problem. Their second piece of advice was to return the ESC to them, so that is my next course of action. They have also echoed heavyleftthumb's comment that these ESCs are a bit crap, Tamiya having decided to replace the previous model ESC with this one. Apparently the previous version was far superior.