After 2 and a half years on the job, I've just done my first ever conservatory.... They're not a very common sight around here! But the roof didn't come up well at all, basically still looked filthy. I was just thinking about how after so much scrubbing it still looked like it had never been cleaned in 30 years when the guy came out and explained that he'd had his tiled roof de-mossed last year and the guys that did it put a tarp over the conservatory to keep it clean while they worked, but that it was windy and the tarp blew off and it got "some sort of black chemical" sprayed all over it. He said he knew it wasn't my fault it hadn't cleaned up well, but did I know any way to fix it for him next time I'm there. No idea what this black chemical was. Any ideas?