Ian,
Though I prefer Ortlieb HB bags myself (and their older Ultimate 5 over the later Ultimate 6 for both the snap-secured lid and map case design...the magnets and newer map case of the 6 have proven not as good for me in my use and I much prefer the Large size for each) in the interest of fairness, I should say there are many who prefer the Arkel line over Ortlieb or Carradice so it is well worthwhile to check them out also. Beyond that, Topeak and Axion have their adherents also so you should take a look at those brands before buying.
There's a number of desirable/beautiful handmade canvas-and-leather alternatives offered by Berthoud, Swift and others intended for the randonneur-touring market, but they are designed to be used with a decaleur (handlebar-stem mounted bracket of a specific type; the other half mounts to the bag) and pretty much have to sit atop a front platform rack. Still worth a look, however.
Some of these other bags are only waterproof if you fit a cover...and that can make access difficult while it is raining 'cos the cover must be removed first...and then stowed later, possibly while it is still damp.
Some additional data points: Ortliebs typically come with a sort of stiff foam internal divider. The 5-series was X-shaped and offered up to four divided compartments in the bottom of the bag The 6-series I also own has a divider that forms into a sort of W-shaped sine-wave making up to three compartments. The X-type can be purchased separately and used in other bags and for other applications For example, I have one fitted in my rack-top pack and it works great there. With any of the bags, it is wise to avoid the temptation of leaving the carry-straps attached while riding in case they could foul the front wheel with disastrous consequences. I ride with mine tucked inside the bag and all is well.
Best,
Dan.