UAG endpoint component cannot be installed/de-installed

Ever since updating UAG to Sp1, we have had a few clients that :

1/ When connecting to the UAG site, UAG tries to initiate the dialogue to update client components, but IE fails to offer the information bar "This website wants to install the following add-in..."

2/ Attempts to manually uninstall the UAG client components return the error "Forefront UAG client components version 4.0.0 cannot be uninstalled. Try again later" 

any thoughts?

Peter