After almost 8 hours of searching/restarts/rebuilds the culprit for me has been found for me.
Trusteer Endpoint Protection which is part of Rapport
http://www-03.ibm.com/software/products/en/trusteer-rapport which banks advise to install for online banking.
If I stop their program everything runs just fine and GSX works.
Trusteer had carried out a silent upgrade which was not obvious but showed up searching through programs affected by restore points.
It appears to control access to windows API's to prevent fraud while using banking
In my search on this forum while waiting for restarts I found this from April 2015
http://www.fsdreamteam.com/forum/index.php/topic,8207.msg90313.html#msg90313 My issue appears to me to be similar to the behaviour suffered by the list of 10 or so programs that cause a similar problem detailed in this old thread.
Umberto said this check was being removed in Couatl 3 beta , perhaps this was reinstated or modified?
Looks like I have a good way of testing that my banking is protected now
