Hey MartyUK -
Lets start with the most probably cause which is an error in the Swarm Agent’s setup and or the DotNET setup. To clear up any possibly confusion make sure that the Swarm Agent from UDK is not started, not in the StartUp folder and not assigned to any Coordinator. Preferably uninstalled. Next you will want to do a clean install of the DotNET Framework. Here is the Microsoft website on how to do that, Microsoft .NET Framework Repair Tool is available
After you uninstall and then reinstall the latest version of DotNET, try building lighting again and let me know the outcome.
Thank You
Eric Ketchum