Well, I made one!
Thank you to Womble for the onspiration, and the detailed instructions!
Mine looks not quite as neat but still quite happy with it, taking into account that I'm pretty much clueless with DIY.
So it took me the best part of 3 hours, plus it turned out to be rather expensive...
Entirely my fault: local builder merchants only does square profile guttering on order. But I wanted to get on with it, and saw some fat grey round guttering standing there in the warehouse, and said 'I'll have that then'. Only when I paid did I realise that this is 'industrial guttering' and 4 times as expensive as the normal stuff
Makes for a bigger and heavier trough, but I don't really need that with my Soays.
Oh well. I'm still rather pleased with it, and I learned a few things making it too