I think I've found the answer:
http://www.kvraudio.com/forum/viewto...er=asc&start=0
"Solved! I can't believe how easy it was, i'm so stupid! Ok, so the problem was that REAPER was not using all cores for live FX processing. In Preferences->Audio->Buffering, tick the "Allow live FX multiprocessing on" and choose how many cores you want. Apparently, it might impare performance, but i'm not sure how. I just asked the poster on the REAPER forums, and I will update this when I get his answer."
It seems a bit daft that Reaper doesn't automatically detect all the CPU's and use them accordingly as standard. Then if this needs to be changed it can be done manually in the 'Options' settings rather than the other way round.