Hey guys - thanks. That's really helpful.
The consensus of opinion seems to be that it's fly ash and water. I do think the coal has quite a lot of water in it. so this would make sense. I could probably improve the cap on the top of the chimney, but the gunk is happening, even when there is no rain or snow. There's more gunk on the cap - so this would make sense too. I had to have the chimney over a neighbours garden, so I'll have to check that it's not dripping into there yard
The flue pipes aren't so well made that I can absolutely choose which way to orient them - especially at the top of the heat exchanger.
I can live with it coming out of the pipes are bit, I was more worried it case it was an indication of something bad. So i'll probably store this thought up until next autumn when i reinstall the stove.