I assume this is a BIOS issue and have tried all possible APM/ACPI settings and have also reinstalled the power management software (deleted the relevant items in Device Manager and let Windows re-detect them), all to no avail. Windows does not report any errors on any of the devices.
The BIOS is Award 6.0 and the version is 107a dated 2000. This is the most recent update I can find. All the ACPI functions seem to work (display and drive shutdown, standby, hibernate, etc.), it's just that Windows doesn't detect when the ac adaptor is plugged in. This isn't a life or death issue but if anyone can shed any more light on it, I would be very grateful.