I have eventually come back to this issue and am pleased to say that I have discovered the (or at least an) answer. The documentation says that to install addins it is necessary to be logged on to the pc as an administrator. However, I am working in a corporate environment and we never (are not permitted to) logon in this way.
The solution.
Create a local user on my pc
Add this user to the administrators group
Login to pc with local account
Install addins
logout
login with network account
Job done.
I have been unable to discover why this works when a networked login that grants local administrator rights does not. nevertheless I hope this answer will help others.
Richard Wilkinson