InDesign CS5 / 5.5 / 6 are not supported under El Capitan - revert your OS or upgrade to InDesign CC2015.
Thank you Derek! I'd hoped it was something straight forward like that!
What was the version of OSX your iMac was delivered with?
The original OS was Mavericks. We've downloaded it again, but when we've gone to install we get the message: "This copy of the “Install OS X” application is too old to be opened on this version of OS X"