What kind of glue (adhesive) is likely to be suitable for a heat-lamp surround?
I believe you're looking to repair the plastic trim piece. For that, I would suggest a model cement that would be available at your local big-box retailer (like Walmart, but, since you're in Aus - Bunnings is Aus only, right? - I would hope you don't have those there), or at a hobby shop.
The good model cements will actually slightly dissolve the plastic at the broken edges, then when the solvent evaporates (in a couple of seconds), the plastic will meld together into one solid piece. You're not actually gluing them together, you are, effectively, welding them together. You'll have a nice strong bond, and if done carefully, you won't even see the crack. If you use something like this, you may not even have to get the trim ring off the light, just squeeze the crack together tightly, apply the cement with a cheap, disposable, fine-point brush, and hold it for 5 seconds or so. This type of model cement is going to cost a bit more, but it will make a nearly invisible repair.
If you can't find that (or don't want to pop for the cost), a &quot;regular&quot; type of model cement (for example, Testors™ is the brand I'm most familiar with from my youth) will work as a glue. I'd suggest that you get the trim ring off the fixture and apply the glue from the back. Use it sparingly - it will take quite a bit less than you'd expect - applied to the crack from the hidden side because it can leave a bit of a mess.
Once you've got the repair made, you have, essentially, nothing but the original plastic trim ring, so there's really no need to worry about heat resistance. If the original plastic was sufficiently heat-resistant, the repaired plastic will be, too.
If, for some reason, either of these glues don't hold it, you'll have to track down the actual brand of your fan and look for docs online to see if they indicate exactly what kind of plastic it's made of so you can get a compatible adhesive. However, I really think that some model cement will do the job for you just fine.