That is interesting to know, actually, not that I need a key right now. My latest one has a little torch in the fob, but the battery doesn't last very long... guess it keeps coming on in my pocket!!
1988 340 1.4 GL, 218k, 5 door (Grey Bess). Gone to CBA. 1991 340 1.7 GL, 64k, 5 door, petrol blue (Deep Blue). 1988 360 2.0 GLE, 140k, 5 door, ocean blue metallic (Blue Bess). 1989 340 1.7 GL, 108k, 3 door (Red Ness). More info here!
The key number will sometimes have been written in the handbook so if you have it worth having a look to see if it has been noted down.
Other option is to phone your local dealer and give them your registration number or the cars chassis number and they will be able to provide you with a new key. That will normally take a 3 to 4 days.
If your local dealer is unwilling to help send me the details and I will be able to get my friendly dealer to do it.