And the difference between bunkai, oyo and henka is ... ?
They are all simpler to undertand than Okuden Waza

Good question though Huw - and one that's much debated on every Karate Forum on the Planet but from the reading I've done the "Overall Term" of Bunkai seems to cover four levels of interpretation of Kata sequences/techniques - with each one being further "removed" from the basic Kata performance techniques (kihon) and the way that I personally have come to think of them is as noted below (in increasing level of complexity/understanding.....)
Bunkai - "straight" application from basic analysis of the kata kihon techniques following kata moves and embusen
Oyo - "Possible" application still following kata kihon techniques but using "interpretive" application of the techniques - e.g. gedan-barai as a strike instead of block - embusen still basically the same but the defence sequences may contain additional techniques to complete the "neutralisation" of the threat.
Henka - "Extrapolated" application of Kata techniques which may appear very different from the kihon techniques and embusen is adjusted as required to make them work.
Okuden Waza - "Hidden/Secret" techniques "discovered" from detailed analysis of the kata (e.g. the "missing/changed bits...)
You may of course have a different interpretation but I would argue strongly that "Bunkai" per-se should not change the kata performance in any way.
EDIT: - other than of course to make the Kata Performance more "Real"

Jim