I've discovered that my problem may have been related to something else entirely and not the fact that the buffers and cache were full.
A few days after my last post I couldn't boot without a disk check running, but the disk checks hung and never completed. I eventually managed to get the disk checks completed manually, and it discovered lots of errors, which were then fixed.
On a whim I tried hibernation again without the below command to empty the cache and it was was successful. I have been hibernating first time ever since without problems, even at times with 30-40% RAM in use, and without the need to clear the cache first, and it is always successful.
So it seems that the initial error was related to errors on the disk and not free memory at all and once these errors were fixed the problem went away. So if you receive the above error, you may wish to check all of your partitions for errors before trying anything else.
Hope this is useful to someone.