Thanks for your help Miriam, much appreciated. Oddly, I have downloaded the rewire installer for mac (Install_ReWire_1_7_Mac.zip) and I have run it, but I when I start the reaper transport from Max5 the audio is really distorted and tinny.
Max support have pointed my in the direction of propellerhead because I have no /Users/Support/Library/Application Support/Propellerhead Software/ReWire/ folder – which apparently I should have… (Also incidentally when I try to get Reaper to host Max5, I get the error message "ad_rewire: error starting audio. Is ReWire mixer running?") Unfortunately propellerhead won’t offer any support as I have not bought any of their products…
Frustrating, I will see if the reaper forum can help.
In the meantime I have shelled out on max4live (& live suite), so I will hopefully this will work better.