Quick update, I came home last night and the yellowness had progressed quite a bit! I don't know if it was a lack of N or nute burn but I flushed just to be safe with ph'ed water, I gave them a foliar feed with biobizz fish mix (high N) to see if it rectifies the problem. The buds are forming nicely and I know that the plants will go a bit yellow by harvest time but this is way to early, they still have at least 2/3 weeks left to go before harvest.

Anyone got any ideas how I can rectify this? I'll post pics later.