Question: I don't know if self-contained units like this exist, but I'd like to think they do, and I just don't know what they're called, who makes them, or where to find them. 

Please bear with me, for a moment. Imagine a lamp, a simple On/Off latching switch, and a battery connected in series. The lamp is powered when the latching switch is in its *on* position.

Is there another switch-type or device that can be added to the circuit, that if pressed whilst the simple switch is *on*, opens/breaks the circuit until the simple switch is *off*. The new device is effectively a latching normally closed switch, but special because it can reset its own state to closed, if the simple switch changes position to off, and current no longer flows.

Ideally, I'd like something with these characteristics:

* 12V 0.5A
* 10-30mm diameter or width
* panel mountable
* LED indicator (not required, but would be nice!)
* ideally looks like a latching switch but resets mechanically.

Any help appreciated.

Answer: 
The function you're looking for is very common in, for example, a sump-pump controller. If the sump starts to overflow (because the pump has failed), an alarm contact closes, and it usually sounds a very loud buzzer. There's a momentary-action switch called "Alarm Silence" that will shut off the buzzer (so that you can fix the problem in peace), but then it will re-arm itself once the original alarm contact opens.

It is easily implemented with a single SPDT relay, as shown below. S1 is the alarm contact, S2 is the "silence" pushbutton, and the relay K1 keeps the load (buzzer) switched off until S1 opens again and the relay drops out. "Power Source" and "Load" can be pretty much anything, as long as the coil of K1 is compatible with the power source.

![schematic of latching relay circuit](https://i.imgur.com/JL1uM.png)
I am unable to get you clearly but I think you are asking about  [push button type switch](http://en.wikipedia.org/wiki/Push-button), a switch through which I wrote this answer(my keyboard). It will remain `on` if pressed and goes `off` when released.

For more information see [wikipedia article Switches](http://en.wikipedia.org/wiki/Switch).