Ben
Seems like a few different issues going on.
To determine the problem with the battery going dead ("flat" as YOU call it
), you need to have a current consumption test done. On this car, there normally is quite a large drain when the car is shut off. It DOES make a difference on this car if the KEY is left in the ignition, you should remove the key when the car is parked in a secure spot such as your garage, when the normal inclination may be to leave the key in the ignition. Remove it and put it on the dash or center console for example. On this car it would not be unusual for the drain (even with the key removed) to be as high as 40 or 50 ma (milliamps, there are 1000 ma to an amp). So, you need to find out if it is alot higher than this what the problem might be, such as a trunk light remaining on for example. The usual way of determining current consumption while at rest id to remove the negative terminal of the battery and putting an ammeter in line between the terminal and the negative cable. You might want to have someone experienced in Mercedes to do this, it would be easy to ruin the ammeter (or worse!) if you don't know the proper order of doing this(for example if the ammeter is hooked up and you open the door and all the interior lights come on you can blow the fuse in the ammeter, a mechanic would know this). Also it may take a bit of time before the car is fully "powered down" so you can see the "real number" of the draw. So that's one issue.
The other issue I think I can help you with, in regargs to the cluster not all working correctly after battery replacement. OK, under the hood area, on the right side (passengers side) back towards the firewall there is a "black box" (control module box it's called), and you need to remove the lid to get in to that. THEN you'll see one module in there that has a clear plastic lid, there are fuses under this small lid. I am betting one of these fuses are blown. This module with the small lid is called the Base Module, or "BM" for short. It protects various modules in the car from voltage spikes and reverse polarities, etc. Don't worry about the fact that not all the fuse ports have fuses in them, just check the ones that ARE there and replace any blown ones, make sure you put them all back in the original spots.
Gilly