I installed 'Portal Stories Demo' but whenever I play it the portals go all funny, they take a while to show what's on the other side and when they do there is this ring around it which has the texture of a portal that doesn't have an exit. Also this carries over to when I play other map packs and when I play the normal story line. Can someone tell me how to uninstall it please?

Oh hai,

I'm sorry to hear the portal stories demo has caused problems for you
How did you install it? Did you create a sourcemods folder like said in the description, you should really read that. If you did: remove the portal stories demo folder from your sourcemods.

If you didn't (what I assume seeing your problems) You should go to steam, rightclick portal 2 go to properties and validate game cache, this might take a while but it should download the files that were overwritten by the demo:

You should be able to properly play it if you have a sourcemod like in the description, if any problems still persist; feel free to ask me again, either here, through a pm, or via steam (username lpfreaky90)

I tried that and it came up with a message saying 'one file failed to be validated, it will be reacquired' or something very similar. Anyway I tried running the game and the same problem with the portals occurred.