Right, as I'm on a deadline I have ordered the most suitable bag for purpose.
http://www.amazon.co.uk/Lowepro-Classified-250-AW-Shoulder/dp/tech-data/B001H33VW8/ref=de_a_smtd
Lowepro Classified 250 AW Shoulder Bag for DSLR, Laptop and 2-3 Lenses - Black
The only question now is, which...