Thanks girls!
I made it this morning and it turned out perfect.
Had the tin of condensed milk sitting in the varoma for 90mins in total. I filled the jug to the 2L line and set the timer for 30 mins and repeated twice. This was to remind me to refill the water each time as it evaporates quickly
I did 2 tins at once. I am making a Salted Caramel Cheesecake Slice and the other tin is being used for a Banoff Pie for playgroup tomorrow.