I'm aware that the titled issue is normally caused by incorrect HTML, but for the life of me I cannot see what I have done wrong. I would greatly appreciate it if anyone would be willing to look over the code and let me know if they spot anything awry.
I've removed the code snippet as the issue was with HTML not provided
I managed to figure out the issue. There was a missing span tag in the previous panel, for some reason this had no effect on that section of the page but manifested as an error on the ajaxToolKit:ModalPopupExtender. How strange.
Thank you to those who took the time to have a look over the code.
Cheers.