Mr Hogg, I have the same problem and have researched it as being Mg deficiency - they even say they're curling up because they're praying for Mg. You also have very purple stems - which I also have which they say is a K (potasium) deficiency. I've not yet done anything about it but plan to get hold of some supplements. I can't work it out though cos they get regular feed which should give them everything they need? I have really good ventillation and temps are nice so i don't believe they are to blame
Actually scrap the purple stems comment i made - it was the pen on your picture...duh