I found the solution! Partially thanks to Rix Yarbrough's youtube link above, from which I learnt that the Border option is respected in interactive viewport rendering when in camera view.
So my new workflow is:
Render without the border to find the noisiest part of the render.
press ctrl + B to draw a render region on the 3d viewport
tweak the materials (adding light path nodes to the bumps etc)
tweak the render settings until ideal (samples, clamping, bounces, materials etc)
copy tweaked preview settings to the render settings (samples etc)
Huge time saver.
For me my scene had a huge number of scattered objects which the viewport couldnt handle, but I needed these included whilst tweaking the settings, so a solution was to select all of the objects that were being used to scatter, and then in the object tab of the properites panel, set maximum draw type to bounds. The particle system then respected this setting and allowed me to display all particles in the viewport, and they still rendered as full objects in the preview render.
Now we just need the ability to view different render passes in preview rendering to better ascertain which ray type is the noisiest. This will speed up optimising the settings even more.