NickW,
AFAIK, there is no explicit law against bringing a knife in luggage, you just have to report it. Even if you don't report it, according to my experience, and as long as it's not a (valuable) sword or something, they won't make fuss about it. My only problem was when I brought 30 something knives (most of them were folding knives) about a couple of years ago. But that had something to do with Trade laws and not of safety concerns. My friend with a 9" fixed blade and 2 folding knives was okay.
If you're visiting a protected National Park, a clear (and translated, if you have any access) Special Permit accompanied by Expedition Property manifest is usually more than sufficient. The ultimate source for the laws are of course, like Brian mentioned, your Foreign Ministry, along with both Indonesian and British Embassy.
------------------
Reynaert