Seems like a problem in the style in that case, could be because of colliding JavaScript files/scripts.
Hi Dave,
After looking at the styles and actually going through and one by one deleting scripts that have always been on the page, I found one that fixed the issue.
It had always been there, but was updated by a third party without our knowledge, so we didn't know to look at it straight away.
Thanks for your default theme suggestion, I forgot I had the ability to easily change the style and compare the two.
The issue was still only limited to IE8, but it's good to know where it was coming from.