Thanks guys.

For users coming after on this one. My method was simply to make the change at a less busy time of day, which probably wasn't even necessary, but always feels safer. I didn't use maintenance mode because that just adds confusion to a change that is pretty much instantaneous.

Results are that we appear to have resolved the issue.

Interesting that on more investigation later yesterday we were also seeing some issues with IE10 on windows 8. Not all variations of this combination but some.

Thanks again.