Looking good man, I've had a good read through your thread,I'm also a UK first time grower using coco, I've got one plant in canna and the rest in plagron. I had no idea at first and was feeding my first plant with just ph'd tap water for about a week to 10 days which stunted growth slightly but not too bad, I read on here to start off with 600ppm but I decided to start with 400 and after two weeks of tweaking I settled at 600 until flowering and they seemed to do well. I've also got a similar coco/perlite ratio as you but mine is mixed 50/50 and I find it's best to use the run to waste method of feeding to reduce salt build up, I use 11l fabric pots and watered with 2 litres of feed since around day 10 watering once every three days until day 20 then started watering once a day after that and now they're flowering I'm going to start giving them 2 litres twice a day so they'll be getting 4 litres a day but about 70% will go to waste, I know it sounds wasteful but autos don't need as much nutes as photo plants and it grows like it would in hydroponics (so I've heard) but apparently this method is supposed to be the best way to grow in coco. Check out my grow I'll be updating it soon