Quite simply I asked this question before being too familiar with the rad controls and what documentation is available.
I found all I needed here under the RadMap section,
http://www.telerik.com/products/silverlight/whats-new/release-history/q1-2011-version-2011-1-0315.aspx
And also I found this which gave me the full answer,
http://www.telerik.com/help/silverlight/radmap-howto-custom-provider.html
When you scroll down to the bottom you see that you need an extra class. One that derives from TiledProvider, and one that derives from TiledMapSource.