Coming July 2012!
UK Sounds will require Silverlight 5 a small, lightweight and FREE browser plug-in from Microsoft.

Click here to install Silverlight on Windows and Mac
What is Microsoft Silverlight?
It’s a small FREE browser plug-in from Microsoft that's very similar to the Flash Player which is used by the BBC iPlayer for example.

Is Silverlight Safe?
Yes, absolutely! It’s a bona fide official Microsoft product. It is offered free with  Windows Live Essentials.

Where can I get more information about Silverlight?
There is a good description of Silverlight on Wikipedia.  You can also visit the Official Silverlight site.