Another Ronan on the forum? Damn, I was sure I would be unique for a while longer!
Ok, so what you want to do is create an ErrorPage type in your theme:
1) in the \themes\YOURTHEME\templates folder, you should see at least a Page.ss - copy this and rename the copy as ErrorPage.ss
2) in the \themes\YOURTHEME\templates\Layout folder - there will be another Page.ss - again make a copy as an ErrorPage.ss
The Page.ss in the ./templates directory is the main holder file. This will contain the <head> tags to which you can add in a bit of Jscript to redirect to the home page after 10 seconds or whatever you need.
The downside to this is that you will now have two main files in the ./templates directory to maintain. Most page types can work off the standard Page.ss one, only requiring a files in the Layout folder, but aside from that, should work fine.
Don't forget to append ?flush=1 to the querystring to see the changes take effect.