Ok, so the lesson today is to never trust your local authority on their water analysis reports. Finally got my pH meter today and did a little test. The shop was out of buffer solution so I made a reasonable test solution by making some fresh distilled water (boiled water in a pot and caught the condensation into a clean dish). Testing the calibration on my pH meter seemed to give sensible results, reading at about 7.01, which is what we expect.
Ok, so the meter is working and calibrated.
Now, I let the water run from the tap until it went cold and filled up a glass measuring cup - tested the pH and got (duhn, duh, dah!) about 6.9! Reading the water quality report for my area, however, gave values of mean pH as 7.8! (7.6--8.2 range) So yeah, never trust anyone - measure things yourself.
So I mixed up some nute solution (1/2 strength plus 1/4 tsp. epsom salt per litre) and tested that -> pH 6.15. Perfect - just right for soil. So clearly, it would seem, that my yellowing leaves are not a pH/lockout problem. Now the mystery remains whether there is still some specific nutrient deficiency or excess, or whether they were just old fan leaves dropping, perhaps a bit quicker on the autos than for a normal plant due to their fast life cycle and very short veg period.
At any rate, I fed at 1/2 strength today with that solution after giving pure water for my last watering. Budding continues to develop and, aside from the old dead fan leaves, the rest of the plant looks vibrant and healthy.
Watch this space - more news to follow.