Gave up on the RS cable and decided to use my ZOOM R16 as the audio interface. After a bit of a Google I got it all working - still the slightest hint of latency, but maybe there is more fiddling to be done. Quite usable.
I was trying to see if it would work with the Rocksmith USB cable - but that seems a tad flaky in Reaper. Everytime i try and select anything except the Microsoft Sound Mapper i lose all sound.
EDIT:
After more fiddling I have got it to work with the RS cable - but I still have some latency.