What is wrong with these girls?

Hi, as you can see from the attached photos, the leaves of one of my plants have taken on an unhealthy yellow/brown colour and I'm not sure why. A little background info:
The plants are of an unknown strain. They were labelled as Lowryder#2 from Irish Seed Bank, but they did not autoflower as expected. After vegging for 5 weeks, there was no sign of flowering and so over the next week, i gradually reduced them to 12 hours light, whereupon they began to bloom.

There are 3 plants growing, the others have healthy looking leaves but much smaller buds. The plant photographed was topped after about 4 weeks of veg.

The plants would get an occasional dose of a simple N fertiliser during veg, and during flowering I have been using a once weekly mild dose of genesis formula bloom and microbase and a once weekly dose of seaweed extract. This browing began about 2 - 2 1/2 weeks ago and since then I have only used plain water, believing it to be an over fertilisation problem.

Plants are in approx three parts soil to one part perlite under 2 x 125W CFLs. Currently at about 5 weeks of bloom. This has happened with some previous grows which have still given reasonably good yields despite significant browning.

Can anyone tell me what is going wrong? I can't tell from any nutrient problem page that I've looked at.

Thanks