Sorry to lock the door after the horse has bolted but i'm not so sure the problem is (entirely) the alternator. I've just had a re-read of your post and you say the battery light is on when the ignition is off, then it goes off when you put the ignition on then all is normal when running. If the alternator was goosed then the light would be ON when the car was running.

From what you say later on sounds like someone has been playing with the electrics and there's a different alternator in there - has it always done that since you had it or is it a new symptom?

The battery light came on Saturday night and the battery has now drained. I've only had the 90 for a couple of months so I don't know it's history but nobody has been fiddling with the electrics.

the same thing happened on a 200 Tdi 110 ex military I had over the summer. The faults you've described are the same and this is most likely due to the diode pack or regulator on the back of the alternator starting to fail. The only way I avoided a flat battery was to disconnect the battery when not in use. I replaced the alternator with an exchange unit from Auto stores ion Malvern for £64 (ATR in Abergavenny wanted £160 for a new alternator!) It is possible to replace only the diode pack for a about £18 if you satisfied that the rest of the alternator is OK.

Jon

Thanks for all your input guys, I'm up and running again.

I took the alternator back and swapped it for one with the plug. Got home and removed the old alt. I couldn't get the pull wheel off so I had to get the bike out again for the 25 mile round trip with two alternators in a rucksac The shop swap the pull wheel for me then back to fit it before it got dark. Charged the battery up for a couple of hours and I'm now mobile again. Thank god for the motorbike as an alternative mode of transport.