Before I take a look inside (or bin the thing) I just
wondered if anyone had an idea what might have gone kaput.


I guess you old charger contains nothing more than a big transformer and a
bridge rectifier. The rectifier contains 4 diodes to convert the AC from the
transformer to the DC for the battery. If one of the diodes goes short
circuit then you will get the effect you are seeing.

If you really want to resurrect the charger you could probably replace the
old bridge rectifier with a new one from Maplins for couple of quid (AR84F
looks suitable). But as modern cars and batteries are more susceptible to
mistreatment I would be tempted to just get rid of it.

Chris