Gnaargh!
So, I'd got this smudge on the sensor, which buggered up a few reasonable photos. I also discovered a downside to the long exposures - the longer you leave the shutter open, the more chance of dust landing on the sensor.
I spent the afternoon at Buscot Park in Oxfordshire, and at one point it clouded over a bit - this shot is a 1 minute exposure and is pretty much everything I wanted to achieve with the 10-stop filter - you get the movement of the clouds and the water smooths itself out.
And a couple of bloody great dust sports.
I really like the picture, but it's been ruined by dust and the mystery smear....