G'day Max --I think your lemon tree may be suffering from ''lime induced chlorosis'' probably caused by lime leaching from the render in your brick wall.If this is the case, bring the PH below 7, mulch with pine bark ( not spent mushroom compost), apply iron chelates and long term, agricultural sulphur, fertilizing yearly with ''citrus fertilizer around the drip line.
The main cause of citrus yellows is either iron deficiency (as above), magnesium deficiency, or nitrogen deficiency.Iron deficiency leaves the veins green with some yellowing between, whereas magnesium deficiency can show a similar pattern, but with a marked ''inverted V'' green pattern and some leaves will turn completely yellow.If it is a magnesium deficiency , still check your PH and apply epsom salts (magnesium sulphate), sprayed and watered in.
Hope that helps.
