Hi Simon,

How odd! This is an absolute first, quite peculiar... Before panicking, I would try the usual catch-all solutions. Have you tried to (in order):
1) restart the browser, or
2) try a different browser (Firefox, Safari, Chrome or Internet Explorer), or
3) restart your PC, try again and then retry 1) and 2)?

If all of the above fails, please let me know:
a) your machine details (operative system and version)
b) the browser(s) that show this behaviour
c) the Silverlight version that you've installed. (on the logon screen, you should get a "Silverlight" option if you right click anywhere inside the white box)

I hope this helps,

Sergio