For those who care....
After much research I built a short-list. The features I was looking for:
* Day pack but light enough for trail running
* Enough storage for some food, rain jacket/possibly long sleeve top for warmth when stopping, first aid kit
* For full day hikes/runs
Initially I was looking at a racing harness/day pack hybrid:
Ultimate Direction Fast packhttps://ultimatedirection.com/fastpack-20/ - too Flimsy
Osprey Talon http://www.ospreypacks.com/us/en/product/talon-22-TALON22.html - too big
Go lite Rush http://www.trailspace.com/gear/golite/rush/ - too big, weird shape
Osprey Rev http://www.ospreypacks.com/us/en/product/rev-12-REV12A.html - probably functionally perfect, but just couldn't see myself trekking around with a bright blue bubble on my back
Racing harnesses: Girvel, Ultimate Direction etc
http://www.bogong.com.au/packs.html?cat=130 - Ultimately too little storage and not adaptable for full day hikes
After trying a few on at shops and testing I realised that most of the racing harness type packs were very light but not very robust. I could imagine tearing them when squeezing through the bush.
My final purchase was the
Osprey Raptor 14.
http://www.outdoorgearlab.com/Hydration-Pack-Reviews/Osprey-Raptor-10This is actually marketed as a biking pack but after much ruminating I concluded it also makes a great day pack. It's tightly compressed when not loaded but expandable to hold quite a lot when required. The bottom pouch holds a roll-out tool kit that I figure is easily converted to a first aid kit, and is very robust, About the only feature it doesn't have is shoulder strap pockets. But I figure if I really need I can buy separately - such as those made by Z PAcks....