Problems with Electrics on W210 E300TD 1997

Last week, the battery died on the car. I changed the battery myself, and apart from the windows needing resetting, the car was running fine for several days.

Suddenly, four days after the battery change, all the electrics in the car went off-line. The key wouldn't unlock the car, the windows refused to work again, the indicator and windscreen wipers wouldn't operate, etc etc. The car itself runs fine, but in order to get into it I had to manually unlock the driver's side door and then the alarm would go off! (putting the key into the ignition wouldn't turn the alarm off)

I've sent it into the Merc garage, but they are stumped as to what the problem could be. I originally thought that the change in battery scrambled/reset the onboard computer system, but that doesn't seem to be the case. There is a huge list of faults (as above, including things like the airbags and more), but even after resetting the computer, the faults are still there.

Could the change in battery and damaged electronics be related, or is it an unfortunate and expensive coincidence? Has anyone else had this happen to them? Advice and/or possible solutions would be gratefully received.

Cheers.

I love my Merc! I want it back, now, please....