Well, I used up three different bags of old charcoal and nothing dripped out of the bottom of the furnace. It took nearly two hours to start burning properly as well. I'm guessing now that it's right on the edge of developing the 660 degrees centigrade that I need. Which does make sense as last burn was quite partial in terms of what melted and what didn't. So next time I'll go with new good quality lumpwood...
On a more positive note, with all the heat my green sand dried out but held the shapes that I pushed into it, so sand casting is looking workable at the moment.