Was not allowed to add this to my other post -accused me of bumping!!
The version of MTN Portal Story on this site definitely has a bug that prevents you getting past the chamber described in my previous post.
Got advice from the people on the Steam Portal forum and found a bug free version at:-http://forums.thinkingwithportals.com/downloads.php?view=detail&df_id=107
Anyone looking to play or having found problems in the past should go for this version.
Have been unsuccessful in alerting the site admin people to this problem - error message appears when you try to post a comment to them!!