Can this be done simply using contactors and switches...

Sure, one pole on the UPS input, one pole on the output and get a contactor with a SPDT side switch to shut down the UPS (they are commonly available)

My problem is not so much where to place the contactors, as that is pretty much a given fact. The problem lies is how to drive the coils of them.

For example to start the system, only the main supply is available for activating any contactor coils. Once it is all started of course, the UPS is there to hold up the power should the main power fail. Therefore, the contactors need to remain closed if the main power source fails, and therefore need to be held closed by the UPS supplied power. At any point though, hitting the push button needs to cause the isolation and shutdown as described.

You use almost any emergency stop button that provides 3 (0r more) NC contact sets.

eg:

These rope switches are good for providing "instant accesss" emergency-off across the entire workshop/equipment room.

You use one NC set to use mains power to hold a mains supply contactor on until the button is pressed.

You use one NC set to use UPS power to hold a UPS output side contactor on until the button is pressed.

You use one NC to enable the UPS until the button is pressed.

The pushbutton locks mechanically in the "emergency" position - preventing the contactors being powered whether the supplies are present or not and preventing the UPS from starting.

Yes, that sounds good, I had imagined something along those lines.

That would satisfy my immediate needs, and I think I can probably identify the right sort of components.

At some point I'm thinking of extending this further, and would for example like some way of automatically triggering a full and immediate shutdown by using a thermal sensor. Although in this case something a little more complicated. I.e. an audible warning to start when the temperature is about 60 deg C, and automatic shutdown at 70 deg C. Does anyone know of a DIN-rail type module that could act as the trigger. Also how do I ensure that the temperature module is able to operate when either or both main and UPS power is available?

I am thinking two 24Vdc DIN rail PSU modules, one just downstream of the mains contactor, and one downstream of the UPS output contactor, with the 24V outputs paralleled. Then finding a suitable DIN-rail module to do the sensing. How does that sound?

An unusual approach for over-temperature of equipment sensing. 60/70 degrees trip points are hardly going to be there to protect occupants.

A more usual one is to sense the operation of the environmental control system (eg the aircon) and/or sense the operating temperature of equipment susceptible to high temperatures and ensure that they are not allowed to go out of specification.

The temperatures were only examples, but to answer your point, the system has no control over its environment. It has forced are cooling, but must be shut down if its gets too hot, prevent gasses or fumes being generated by over heated lead-acid batteries in the UPS, and to prevent any conditions that may lead to a fire. So yes 60/70 may sound high, but we're talking internal temperatures here, and the cabinet will at most times be operating in an unoccupied space.

Go with a DC contactor, plug a wall wart in utility power and another one in UPS power. Use a couple diodes to isolate them from each other but common to the coil and put your N/C thermal switch in series with the coil.