What a difference a day makes.
I had put the mid corner oversteer in the Ruf C-Spec at Okayama down to my driving: too much trail braking up to certain corners leading to not having enough traction on the rear tyres when I tried to accelerate.
I had gone through all of the setup options I thought would help, but after reading through some forum posts it appeared adjusting brake balance forward might also help with it. And in fact on using the maximum front balance allowed - 54.3% - the car was transformed. Instead of being hesitant mid corner with throttle application I became much more confident. The 1.33.5 that seemed distant from the current personal best of 1.34.5 became a 1.32.9.
And that time is with practically race fuel, and without having pushed it to the limit, also without having a feel for the width of the car yet. With more experience and a low fuel tank I'm confident of a 1.31.5 time.