Seems I'm the idiot then, missed the "gateway" bit... so this basically sits between something providing VNC/RDP and makes it accessible with just a web server?
One would hope Tesco would cover any costs incurred by customers that are a direct result of this. I know banks in the past have cancelled non-arranged overdraft fees when they've screwed up, but that's easier to do as it's cancelling a charge rather than actually paying money out.