Trust me, in these forums, no one wonders why anyone wants to grow the tropics, in or out of them. The question is more likely why wouldn't you.

For the metrically challenged, the pot is 20 inches and the plant is 5 ft (yes, I had to look it up).

Not an expert either, but not familiar with that cultivar, It may be incorrectly named or named something else in the States.

Maybe some silly questions but, did they fruit? The main stalk will die off afterwards. Are there pups?, are they healthy?

You should start with what it is not.
Light (I would suspect too little if anything, including rapid shifts in quantities of light)
Infestations (above and below ground)(are there a family of Taz Devils living in the pot?)
disease
temperature
water (not just how much you are watering, but how much is in the soil, bananas don't swim well)(what is in your water?)
Soil PH
Nutrition (too much too little)(what fert. are you using, how much, how often?)
Are the roots happy and healthy? (root bound)
What has changed since they started dying off?

How are they dying off, are they fading, burning, splotchy, or just wilting away?

All too often plants die off from too much love.
If you posted a picture, it would help. I sure others would also love see what a conservatory in Hobart looks like.

There is some pretty serious talent in these forums (the banana forum is a little less active), if you give these people a little more information they may be able to help.

Good luck.