Leaves going black and white patches are more likely fungal in origin - it may be you're keeping the plant too wet, it should only be watered when the surface of the compost feels dry to the touch, but not so dry its shrunk from the sides of the pot. My first thought with 'white patches' was powdery or downy mildew, but neither is visible in your pictures. More info in the link, including care (although its aimed at growers in the USA)

http://www.mykitchengarden.info/2015/08/growing-curry-leaves.html

Insecticide is a bit overkill for one greenfly! Easier and more environmentally friendly to simply squash it if you feel the need to get rid. Best not to spray with chemicals if you plan to use the leaves in cooking. Keep your eyes open for young greenfly - most likely clustered under a leaf. If you catch them young you can just wipe them off with damp tissue.

Bamboo I think you can just about see the white patches on the topmost leaf in the second photo. It does look like mildew.

Yes, got it under magnification, thanks Stera - Gardenlover, if its powdery mildew, mix up 1 part cow's milk to 9 parts water and spray the plant thoroughly, including under the leaves, with the solution - repeat every three days if necessary. Moving them to the conservatory was probably wise, they need more airflow to help prevent fungal problems - change your watering regime too, if you've been overwatering or keeping them too wet, and ensure the drainage holes in the pots are not blocked, and empty out any outer pot or tray 30 minutes after watering so the plant's not left sitting in water.

Thank you for your advice, sorry for late reply.
I pruned the infected area and left the stick only for it grow again. my neighbour advised it was thrips as she lost a few plants before discovering it. I am not very happy informing but i did use pesticide which kills "larvae and eggs" i have see it has new leaf coming hope the infection has gone. fingers cross.

will keep you posted.

thank you once again.