Many thanks for the clarification David.
Sorry for the misunderstanding; I don't have a 'turn everything off' macro (am I missing it?) - I was referring to the 'Turn everything off' button in the following screen capture snippet:
Maybe I broke that button's functionality when I hacked DWC to display a larger temperature graph - I don't see how, but it is a possibility.
I've just played again with that button and notice it's completely non-functional (i.e. with and without first cancelling a print) - If I turn all heaters on manually, then click 'Turn everything off', it isn't functioning.
Additionally, I just tried cancelling a print (with bed at set temp, extruder almost at set temp) and restarting the same print only to discover another problem: the extruder temperature is never re-set to printing temperature - the extruder head moves but cold extrusion protection is apparently kicking in to prevent extrusion. This probably needs more testing and verification, but will need to wait a week until I can play again.