Some success!
Reading this post and following the instructions ([RELEASE] Enhanced Wemo (Connect)) I managed to get the problem switch working.
After uninstalling the old smartapp and installing the “Advanced” version of Wemo and the device types, I had to reboot my trouble switch. It was then detected by the smartapp but I started getting “Network Id must be unique” errors. Uninstalling and reinstalling the smartapp from the ST app solved this error.
I have been running 5 Wemo light switches for the last couple of days without incedint. In fact I installed the fifth switch today.
Belkin seem to have improved things on their end with new firmware and the Wemo (Connect) Advanced smartapp also seem to be working great.
@ule thanks for the assistance!