OK problem solved - I uninstalled JDownloader, cleaned out the registry, rebooted and then reinstalled. Works fine now so thanks for your inputs tom1807 and thanks also to catymag for the new links. Much appreciated!
Be aware that the full installation of JDownloader installs browser hijack software (Facemoods) that changes your default search engine and home page in both IE and Firefox. It will also do that in Google Chrome. It can be uninstalled but one has to ask why the programmer sees fit to hijack your browser at all?
I found it necessary to delete all references to it in the registry as well.