Hello Stuart Hemming,
I tried to reproduce the problem locally but I wasn't able.
Can you try to run modify one more time and make sure that all Visual Studio 2010 instances are closed.
If the Telerik Visual Studio Extensions are still missing then you can install them using the Visual Studio Online Gallery through the Extension Manager in Visual Studio 2010. The VS Extensions are called Telerik WebUI VSExtensions
I hope that the provided information will help you to solve the issue.
the Telerik team