Just an update.
I discovered what was causing the problem and I thought it might help others to know.
After much trial and error deleting plugins and patches, I tried disconnecting anything with a USB connection. I realised the MS files would load fine. I finally tracked it down to my EWI5000 which was connected via USB.
All I need to do now is make sure the EWI is not connected when I open a MainStage file, then plug it in after its loaded.
So pleased to have my SL back again.
Hope it helps someone.