Ok, I've done all that, thanks Treed.
FYI, the only apps I have installed in the last few day were Remember The Milk and DiskCatalogmaker. That last one I had my doubts about - it installed the app into the applications folder but nothing appeared in launch pad. It also works at quite a low level presumably to be able to catalogue hard drives. I didn't catalogue my main hard drive with it, just a few external drives I keep my film making archives on. If it didn't come in through those, what other way could it have got onto my system?
Thanks for your help.