Thanks for the very informative comments, I did have the PH at 6.5 which I think has caused a lock out, I treated it like soil

I started flushing it yesterday with a PH of 5.6-5.8 and the run off was around 6.3 which I didnt think was too bad. When I flushed I added a very very small dosage of liquid nitrogen so hopefully it will help.
Where is best to purchase cal-mag or epsion salt, a chemist possibly?

I'll take another pic soon, it is worsening.

Cheers guys,