Thanks to everyone who has tried to help - it appears I may have hit on a solution.
In the RME TotalMixFX app there is a “reset total mix” option and it was suggested to me on the RME forum I should try that. After doing so Cubase seems to be working without issues at low sample buffer sizes (hooray).
Since installing the RME device on my system the only thing I have done in the TotalMix window is move the level sliders up and down occassionally. So I am a bit surprised (to put it politely) that something in this window would have such an impact on the behaviour of the interface.
If anyone else comes across similar issues - in the TotalMixFX windows go to Options>>>Reset Mix>>>Total Reset and then see if the changes the behavior of your interface.
I will report back if this issue flares up again, thanks again to all who tried to help.