Hi. You could melt the jelly cubes with half the quantity of boiling water, and top up the rest of the quantity with cold boiled water. Then pop it in fridge and it should be set by tea-time.
I put mine in pyrex jug with a couple of tablespoons of cold water.put in microwave on high 2 minutes. then make up to just under a pint with cold water.
I put mine in pyrex jug with a couple of tablespoons of cold water.put in microwave on high 1 & half minutes. then make up to just under a pint with cold water.