Jamie,
It is possible you have either blocked access, or removed the commands that allow HTTP.
http server enable
http {your inside subnet}{and mask} inside
also I think you need
asdm image disk0:/{asdm filename}
asdm history enable
I think you need the first, but the second one is optional. Try it without these first, and see if it works, then try it with them.
Hope this helps.
Regards,
Iain