Iceman, I finally did it!
It took 5 hours of googling, trawling forums and rewriting HTML and CSS but I got it. I dont know why they dont just have a spoiler tag as standard. Its so silly. Anyways, I have managed to get [/spoiler] BBCodes working. There isnt a button for it yet so you will have to add them manually, I will work on that once I have had a kip, but it is something at least. It should look like this
[spoiler]spoiler[/spoiler]
When you click post, it will look like it hasnt worked, but thats because you will be in a preview mode. Once the page is refreshed, the tags will be active