Chop both sorts of chocolate small, or use chocolate buttons made for melting, and then put into a heavy-based pan to melt with the butter and syrup over a gentle heat.
Put the biscuits into a freezer bag and bash them with a rolling pin to get big- and little-sized crumbs; you want some pieces to crunch and some sandy rubble.
Put the Brazil nuts into another freezer bag and also bash them so you get different-sized nut rubble.
Take the pan off the heat, and add the crushed biscuits and nuts, whole glacé cherries and mini-marshmallows. Turn carefully to coat everything with syrupy chocolate.
Tip into a foil tray (9¼ x 12 inches), smoothing the top as best you can, although it will look bumpy.
Refrigerate until firm enough to cut, which will take about 1½–2 hours. Then take the set block of rocky road out of the foil tray ready to cut.
Push the icing sugar through a small sieve to dust the top of the Rocky Road. Then, if you like, add a sprinkling of edible glitter for some festive sparkle.
With the long side in front of you, cut into it 6 slices down and 4 across, so that you have 24 almost-squares.