Is a water-butt like a cistern? Rain water? That's your problem. You're in UK? Industrial coal-burning leads to acid rain, rainwater has no minerals to buffer it back in range, your lime in the soil gets used up, your plants get sick and die.
The up side is you are cloe to harvest. The down side is they are pretty bad. Go back to tap water. Then test your runoff pH and adjust the water so that when you water, the runoff comes out 6.7. There's a sticky abotu flushing at the top of problems.

That plant shows classic sour soil and rootbound symptoms.