Seems that given your situation a fully automatic emergency generator is your only option, unfortunately a somewhat expensive option.

We have a rather splendid insert wood burner in our fireplace, but the fan associated with it is hydro driven. Likewise our oil fired forced air furnace is - i'm pretty sure - hydro driven (pilot light etc). I have been considering a backup generator for a while. Anyone know a good one? Not crazy expensive as would be in the garage or outhouse so doesn't need to be particularly quiet, I guess just powerful

We have a gas stove which replaced a wood stove. We've a back up generator which we've only had to use a few times thankfully. Don't really need it for heat but the septic has a pump.

Depends what you envisage running off the generator. For example, will you require to run say, a well pump for water which would require a capability of 220V/30 amp output. Would you hook it to a generator breaker panel and transfer switch? If not and would simply run an extension cable to run a a sump pump, furnace, freezer etc, then something around 2500 to 3500kw output (120V) would likely do for you. Cost around $600 or so. Suggest you speak with an electrician with regards to what your expectations are and take it from there.

We have a very simple setup.
A 3kw generator stored in garage but moved outside under shelter when needed. Connects to two inbuilt extension cables that feed sockets built into wall next to existing sockets next to furnace and other next to TV/internet in living area. When needed ,fire up generator, connect to cables and change plugs from mains to cables. Very cheap and practical. Eliminates any possibility of cross connection from mains to generator, both systems separate. Generator second hand cost $200.. no tax. Has enough oommph to service furnace, entertainment and microwave/coffee maker.
Daughter has 8kw connected to transfer switch panel. Very good but quite expensive, but does service most of house.

Our heat went out overnight, the cause was the battery failing on the programmer. I was worried for my heat sensitive animals. We have an electric fan heater in the bedroom and a generator that's never been out of the box. On the shopping list is hardwares for the animals if this happens again. If it's long term, we'll be off to a hotel!

My cold blooded animal friends are my biggest worry. The gecko I could in most places just go to the car with him in a box and take a drive, but in this building whatever genius designed it they made it where if the power is out there is no access to the garage and cars.

The fish can't move them, water does take time to cool, but anything below 73F and things start to go south as the fish are from very warm regions with water temps pushing 80F.

If the place was insulted well, wouldn't be so bad, but turn off the heat and the temp drops pretty quickly.

We have a similar setup for our furnace and works well for us. We cut the power cable to the furnace and filled the gap with a plug and socket. If we have a power cut we can pull the furnace power plug out of the socket separating it completely from the house power supply and then plug it into an extension cable connected to a small cheap generator ( but not as cheap as $200 nice!).

Could you get one of those power packs for a back up for your aquarium, perhaps, and get a sheet of insulation that you could cut and put around the tank (stick it together with a bit of gorilla tape)?

MotoMaster Eliminator PowerBox® 600 | Canadian Tire

MotoMaster Eliminator PowerBox® 2000 | Canadian Tire <- has 3 outlets

These power boxes whilst very convenient don't usually provide a lot of total stored energy. A deep cycle battery or even a normal sealed car battery (for only very occasional use) with an inverter would probably work better and be cheaper.

This?

https://www.costco.ca/Duracell-400-W...100230281.html

And would a car battery stay charged sitting in storage over time? Or I suppose we could buy a battery that fits our car and swap the battery out from time to time to keep it charged?

Unfortunately in a power outage we have no access to our car as there is no way into the garage without power.

The heater is 100 watts and the water pump around 4 watts. The heater doesn't run constant however, comes on and off as needed.

That inverter would provide enough power. I'm not sure how long a battery would stay charged but practically over the long term you would probably really need a battery charge; ideally a "smart charger" that you could just leave connected and forget about until you need a charged battery. The first question you need to ask yourself is how long would you want to be able to run the heater and also would you want to be able to run a light or anything else etc off it as well? Once you have a rough idea of demand you can then try and calculate your battery needs. To give you an example a 100 watt heater running for 40% of the time would flatten a decent deep cycle battery in about a day.

My stepson is in the same position.

He has a log burner in the basement (vented to the main floor) and lots of wood. He also has a sump pump and a well.

To compound problems, he's away a lot and for extended periods. A mate of his is keeping an eye on the place but you never know. Today would not be a good day for an outage. It's well above zero and raining.

I think I'll tell him to give his mate a nudge.

We should probably invest in a generator.

Wrapping a duvet around the tank also works. I worry more about needing to oxygenate the water,

I'm not sure how much / often to agitate the water

Look on the bright side. At least you'd have a food source in a prolonged power outage.