Wow, awesome suggestions all of you.

I ended up having to do it all in 15 mins as a lovely girlfriend popped over from Perth....

I bought a packet of double-chocolate gluten-free biscuits from Maxi
then blended them with 1/3 of a tin of condensed milk
smooshed it flat and put it in the freezer
melted the 200gms block of white Plaistowe cooking chocolate
mixed in some pure icing sugar (must be pure), peppermint essence and 3 drops green food colouring
stirred it all together
then smooshed that over the top of the biscuit mix and put it back in the freezer
Then melted the 200gms block of dark 70% cocoa Plaistowe cooking chocolate and poured that on the top.

MMMmmm yummo! Peppermint chocolate slice in 15 mins!