4 punnets (approx 800g) of blueberries.
2 bananas (medium, a little on the soft side is good)
100 ml single cream
300 ml double cream
200 g brown sugar (just shy of a cup) (can use ~125 g refined sugar instead)

Blend the blueberries and bananas down into a fine mix.
You can add a tiny bit of apple juice or other juice to make this easier 
if you need to.

Pour the blueberry/banana mix into a saucepan, and place on moderate heat.
Reduce the mix down till you have a thick, jam-like paste, adding the sugar 
early on in this process.

You're looking to render down to about 300 ml worth of stuff.

Be careful, if it boils, it's going to spit sticky, sugary napalmy death
all over the place. It burns.

Allow to cool.

Mix in the cream (both kinds) to produce a thick, smooth mixture.

Pour into maker and follow instructions.