Thanks everyone for your suggestions. After considering around 50 backpacks (not kidding) and spending far more time looking at backpacks than any sane person with a toddler really has to spare I've ordered the Timbuk2 Division Jet Black Static, with Timbuk2 suggested by @gehenna and @iamsammajor. It was on sale on their website for NZ$135 plus $10 shipping, cost me about $150 or so I guess. It's 22L, business friendly appearance, takes a medium sized laptop, and has good internal organisation. I almost went for the larger authority, but I just don't need a 28L backpack. Their older Q model is a great price on Amazon, but I didn't like the laptop pocket.

 

The Tom Binh Synapse 19 / 25 was my next choice, as suggested by @Geektastic. Looks good, lots of organisation, comes in orange when it's in stock, and would probably last many years.

 

I liked Macpack a little, but their website gives little detail, their stores have only a subset of the bags. Everki make nice bags, and I like the orange lining, but theirs were either too small or too large for me. Eagle Creak Wayfinder 20L looked good, but it's so new there's few reviews.  Recycled Batallion looked good but lacked features.

 

Pretty glad I'm finally done with that job!