Hi All
Although I have solved this problem an interesting issue I came accross when using System.Diagnostics.Debugger.Break(); in a plugin to launch visual studio under windows 2012, when I moved to a different machine under windows 10 with visual studio 2015, windows would report that the program had produced an error but not offer to launch the debugger a quick hunt on th web showed that changing to System.Diagnostics.Debugger.Launch(); would get the result and it works.
I am still not sure why break stopped working though.
I have posted this in case anyone gets the same problem.