Hi Hugo,
Glad the videos are working for you now... probably fixed it for some others too.
What I did was add a bit of code to the YouTube button that specifies that the video we are embedding is a Flash Player video... Interestingly, the forum seems to re-write the code into the page, using what I've set up for the YouTube button, every time the page is viewed... that's why my change affected old posts, which was confusing me. Learn something new every day!
It must also be to do with the way your PC is setup (probably in IE), as I have always been able to see these videos... still, as I said, there will be others out there with the same setup as you who would not have been able to see them. So thanks again for your patience with me, it was a useful exercise.
All the best,