I think I will go with the following excerpt:

1. OLDER LEAVES CHLORTIC, usually necrotic in late stages. CHLOROSIS ALONG LEAF MARGINS EXTENDING BETWEEN VEINS PRODUCES A "Christmas tree" PATTERN. Veins normal green. Leaf margins may curl downward or upward with puckering effect. Necrosis may suddenly occur between veins. POTASSIUM or CALCIUM EXCESS can inhibit uptake of magnesium... MAGNESIUM DEFICIENCY
When the external magnesium supply is deficient, interveinal chlorosis of the older leaves is the first symptom because as the magnesium of the chlorophyll is remobilized, the mesophyll cells next to the vascular bundles retain chlorophyll for longer periods than do the parenchyma cells between them. Leaves lose green color at tips and between veins followed by chlorosis or development of brilliant colors, starting with lower leaves and proceeding upwards. The chlorosis/brilliant colors (unmasking of other leaf pigments due to the lack of chlorophyll) may start at the leaf margins or tips and progress inward interveinally producing a "Christmas" tree pattern. Leaves are abnormally thin, plants are brittle and branches have a tendency to curve upward. Twigs are weak, subject to fungus infection usually leaves drop prematurely; plant may die the following spring.

I guess I give it a good flush and start of again with a weak flowering fert.

Thanks bonbon