These chargers often lead a hard life, working in dusty, hot and noisy conditions, so I guess many of these fail in time.
This Bosch unit is fairly common among Bosch DIY drill sets and this one had died catastrophically. With the power applied, this one refused to give the slightest charge to a drill battery, once plugged in.
After some basic testing, I decided to change four components which would have caused the other to fail in a ‘domino effect’. The cost of the replacement parts was just shy of £10, but definitely worth saving since second-hand units seemed to be changing hands for £40 on eBay, with their condition largely unknown. The parts (two resistors, MOSFET transistor and diode/ transistor) were readily available online.
I recorded a short video to help others who might have a similar problem with theirs…