Sorry for the late reply, life took over for a while.
I had already done as you said but was getting nowhere and eventually I left it until today. This time I deleted both copies of Opera, installed and portable, along with any other references to Opera I could find, only one actually. SP6 was still allowing me to install and uninstall the portable Opera add-on, even though I no longer had Opera installed anywhere. I re-installed Opera and this time the SP extension installed correctly, and at last the checkmark, which had been saying the (non-existent) portable Opera extension was installed, disappeared. I decided not to install another portable version.
All very strange, and no answer to what was happening, but at least it is sorted out now.