I'm not 100% sure how old the plant is but I think it's a few weeks old.
The soil is the best you can get in the UK and the pH is usually between 5.5 and 6.5. My friend has an electronic pH meter (see photo). How would he measure the pH or the run off using this? Does he just water it, collect the water that collects at the bottom and measure the pH of that using the meter?
When he transplanted it into the soil he started to use a weak propagation nutrient (
http://www.greenshorticulture.co.uk/Propagation-23/Propagation-Nutrients-302/Formulex-512.asp)
Maybe he should stop using this?
Okay - yes, just collect the water at the bottom, and pH that using that runoff water. I recommend watering with a pH of 6.5 - 6.8. But you want your runoff between 6.5 - 6.8, so you may need to adjust the pH going in depending on your soil.
Most soils contain enough nutrients for 3-4 weeks (sometimes more) and you're probably just burning the plant a bit -- although, burn usually shows some browning on the edges which your plants aren't displaying... which means that your pH may be creating a lockout (where the nutrients are unable to be absorbed).
The soil looks VERY short on perlite and/or vermiculate which is necessary for proper aeration. The roots require access to fresh air... if/when you transplant again (or with your next plants) I would recommend using about 40% vermiculate or perlite (or a mixture) and 60% of whatever soil you are using.
How often are the plants being watered? (someone mentioned over watering)
