Many thanks for your input!

Yes, the query is related to one of our Shared servers, so I have no control over what software is installed and also not the full details of what weird and wonderful modifications are running.

The server is live (with live client sites), hence why I'm treading very carefully here. First step is assessing the most common software types (Zen Cart, Magento, WordPress, Joomla, OpenCart etc) for 5.5. compatibility which appears to be all good.

We probably will hold off with MySQL 5.5 for a bit longer (until after Xmas) and also run more tests on a non-client server first to assess.

Many thanks for your advice, that's been very helpful!