Well when I said "blisteringly fast" I should have said "very fast, but give it a week or two and we'll get faster"...

I've since found a way of improving the performance of my solution... and I think Dimitre has improved his. Watch this space, because neither is quite ready yet. I'll stick my neck out and say once it's done, mine should solve any board pretty much instantly. (I'm going to regret saying that, because mine probably won't but Dimitre's will...)