Thank you, all, for your help. Unfortunately I've now got myself into a real mess. Having failed to get round the Windows PC's manipulation of the upload, I tried again with an Ububtu Linux laptop, and it worked. The box started correctly, the Vix entry reappeared at the bottom of the main Setup menu, and the box received broadcasts normally.
Thrilled by this success, I then went to the image manager and launched the upgrade to version 019. I pressed the Power button to start it off, but very soon the box became stuck on the GigaBlue splash screen (on both the LCD and the HDMI output) and progressed no further. I've since tried installing 019 from a USB stick, but that did not progress either. So I'm at a loss to know what to do.
Thanks, twol, for pointing out another plugin that is crashing, but I'm afraid I don't understand which one it is :-( Is it something from the plugins menu (which, alas, I can't access at present) or is it hidden? Perhaps that's just an academic question at the moment.