This is a 1994/5 Disco 300Tdi... drove well between Swindon and Bristol airport last Friday, didn't move Saturday, Sunday morning it was very sluggish to start. Voltage (from 3rd party display in car) read 8.something volts while cranking, but it did start! Ignored it until last night (Monday), voltage across battery is reading 4.something volts. Popped in 'spare' (old) battery this morning, started up, and in-car display is showing 18V! Stopped, checked battery and it showed 12.2V.
Thinking the alternator is faulty and overcharging, which would cause the first battery to fail? Didn't notice voltage display at time to see fault. However, when I was poking around today I felt the middle terminal (D+) on alternator was hot. Car hasn't run for more than 2 minutes, and that was 6 hours ago!
Time for a new alternator?