CS7.2 is "Adobe InDesign CS5"
You could install a Creative Cloud trial of InDesign valid for 30 days, and export the file to IDML from there.
Thanks for your response Eugene. I'm confused by "CS7.2 is Adobe InDesign CS5" though - I have 5.5, surely I should be able to open any documents saved in CS5?
Downloading the CC version is my backup but didn't want to waste a trial for this when I might need it in future. Running out of email addresses!!!
Eugene is also confused....
CS7.2 does not exist, and the error messages were written before the new CC naming system was adopted. CS7.2 would be equivalent to CC, version 9.2. You could use the trial version of either CC or CC 2014 to make the .idml file if you haven't already installed them (someone must have been playing with CC).
InDesign CS5 (7.0) (codenamed Rocket) released April 2010...
This is why I am confused lol
Yes - original advice still stands - install CC2014 and do the conversion to IDMl.
Found an acquaintance with CC2014 in the end, but thank you both for the advice.