I'm posting this in the hope that it helps someone else.
Thanks to all for your advice on this. In the end I took just the kit lens (Canon EFS 17 - 85mm), my Canon 430 EXII flash, a compact tripod and my netbook computer.
I was concerned about weight and in the end it was justified. Melbourne Airport (Australia) is really particular about weight.
I wish I had more reach as I've got lousy photos of the orang utans in Borneo, I used the external flash once, though I should have used it more, and I used the tripod once and want to replace the head with something more secure.
Occasionally, I struggled to keep my shutter speed up. The Canon 450D gives disappointing results at ISO 1600.
I didn't miss my ultra wide as I'm just photo stitching - the Canon supplied software is pretty good.
So some lessons for next time perhaps...
- An 18 - 200+ might be nice
- The 430 EXII flash was a bit of a waste of weight - with batteries, it had to be a good 500g. Perhaps something smaller but more powerful and flexible than the onboard flash would be more convenient.
- If I insist on going without an external flash, better high ISO performance would be handy.