I must be getting very complacent. I updated Filezilla and faced with what looked like the usual T&C page in the installer I clicked Accept. The second page of the install wizard asked me to install some 3rd party software which I declined and then it said it was downloading my software..

About five minutes after installing, a window popped up saying it was scanning my machine. It was something called “PC Utilities Pro Optimizer Pro” which Filezilla had installed. Luckily, I only had to uninstall it in the usual manner to get rid of it.