Found the culprit, though not the root cause…
In C:\Program Files (x86)\Steam\config\steamvr.vrsettings there was a line in the steamvr block saying "maxRecommendedResolution": 810.
Removing that line when SteamVR was closed looks to have fixed things.
Not sure quite how that setting got in there - I was playing with maxResFactor in cxrDeviceDesc at some point but then also played with the Video settings UI in StreamVR trying to fix it. I’m happy for that to remain a mystery now I can get full res from my local setup :)