From the conversations I had with the OC, it’s very unlikely they will agree to pay for the installation of a triple-glazed window.
They might if the alternative is to move the water heater.
The first thing you should do is start proceedings at Fair Trading for mediation with a view to taking action at NCAT under section 232 (2), failure to fulfill their responsibilities under the Act.
And bear in mind, your job is not to come up with a solution; the OC’s job is to resolve a problem with common property. If they say it’s not common property, tell them you will be removing it and replacing it with an indoor heater just for your home.