This is a large bag!
I am not quite sure why Billingham made it part of the Hadley family as with the conventional buckle straps rather than the clog bolt fasteners it seems to be to straddle the style of the Hadleys and the 5 series bags, though it is slimmer than the latter. It is nearly the size of the 445 - taller but with less depth from to back.
SP50 shoulder pad and Billingham "dust bag" included/

Some info from Billingham:

Larger in all dimensions than our ‘Hadley Large’ camera bag, with additional features such as a 16” dedicated laptop slip and rear pocket. Can carry 2 DSLRs with medium-to-long lenses (such as 70-200mm lens) attached plus an extra spare lens or be used as a great overnight bag. Made in England with 3-layer waterproof fabric and real leather.


External dimensions
W450mm (17¾”) x D180mm (7⅛") x H320mm (12⅝") (excluding top handle)

W450mm (17¾”) x D180mm (7⅛") x H340mm (13⅜") (including top handle)
Internal dimensions (Main Compartment)
W400mm (15¾") x D135mm (5⅜") x H280mm (11") (inside padded insert)

Please note this bag and its padded insert are flexible - especially front to back. The bag can comfortably bow out to a 'D' measurement of 140mm (5½") at the bottom and 170mm (6⅝") at the top (at the centre), at a stretch it can go out to 200mm (7⅞").
Capacity (Main Compartment)
10 litres (0.35 feet³) (With padded insert)
13 litres (0.46 feet³) (Without padded insert)

Capacity (Total)
15 litres (0.53 feet³) (With padded insert)
18 litres (0.63 feet³) (Without padded insert)

Weight
1.64 kgs (3.62 lbs)

There is a pretty good review here:

http://joerivanderkloet.com/the-billingham-hadley-large-pro-review/

I would love to keep this bag but I have four other Hadleys (two Hadley Pros, a Hadley Large and a Digital) and if I want to carry this much gear these days is needs to be in a roller bag.