Specifically, the boiler-side ball valve is connected the wrong way round. I purchased a pre-made Pegler set of valves and flexible tube and noticed it was pre-made in the same incorrect way as the ball valve I was replacing. These small ball valves have a "flow direction" arrow on them but are actually, unlike some taps[1], generally indifferent to flow direction. What the arrow does relate to is the side which best withstands static pressure when they are closed. Theory and some practical tests suggest that pressure from the side at the back of the arrow forces the ball against its seating and reduces the chance of leaks. When used in a filling loop, the flow direction is towards the boiler, but only for a tiny fraction of the life of the valve while the boiler/CH circuit is actually being replenished. Most of the time there is little or no pressure in the actual flexible hose and the valve is closed against the pressure from the heating circuit, of the order of 1 or 2 bar. Therefore, for reliability the valve should be mounted against the nominal flow direction with the back of the arrow towards the heating circuit the arrow *towards* the loop. It seems that both Pegler and my plumber think differently, which is perhaps why I am having to replace the valve because of a leak round the spindle after a year of very occasional use. I am aware that they tend to leak when unused for a long time and then turned, but perhaps this is more likely if lime-bearing water has got into the seal because mounting them the wrong way round allows a slight space to develop under pressure.
[1] Taps with a washer on a rotating jumper behave oddly or completely fail to open when operated in the wrong flow direction. The hole in a ball valve is symmetrical and there are minimal forces in either direct on the ball when it is opened.