Which size McVities biscuits, there are more than one, and where does the second can of condensed milk fit in?

For the biscuits, I would assume the standard roll size of 14oz/400g, and for the condensed milk, I would guess that you're using them both at the same time. But you might want to consider sending a message to the recipe author (by clicking the envelope button on their profile page: http://food52.com/users/35293-cristinasciarra) to ask for clarification.