Here's the up-to-date scorecard after adding a bunch more bags and tweaking the weightings:

1. Observ Slim
2. Ogio Newt 13
3. Booq Cobra Squeeze
4. OGIO Covert
5. Promax Stage 13" - Can't get the nice black
6. STM Aero
7. Chrome Skarbek - Thanks to /u/korgothwashere
8. Zero Halliburton Zest Slim
9. Kensington Triple Trek Slim - Bad reviews on reliability
10. Booq Boa Squeeze
11. Solo Pro 15.6" - Bad reviews
12. iblue waterproof laptop bag - Never heard of them, doesn't look to have much protection
13. Tucano Expanded Work_Out - Not quite sure about the material
14. Topo Designs Rover - Cinch closure could be a deal breaker
15. Knomo James
16. Timbuk2 Alcatraz - Love my custom design, but unfortunately just too damned big
17. Riutbag R10
18. Be.ez LE Rush
19. Brenthaven ProStyle Slim
20. Timbuk2 Octavia
21. Chrome Bevin - Thanks to /u/korgothwashere
22. Arc'teryx Blade 6
23. GoRuck GR Echo
24. Victorinox Luggage Altmont 3.0
25. Crumpler Baked Bean - Thanks to /u/keikun13
26. Cocoon Slim

With booq having a 20% voucher until the end of December (i.e. today), and 30 day refund policy, I've splashed out on a Cobra Squeeze since - apart from the price, and it perhaps being heavier than I'd like - it looks to be spot on what I was looking for.

Since there seems to be a dearth of reviews and pictures for that bag, I'll try and fill that gap a little when it arrives.