David,
I tried this theory again with the key in every position with the exception of running but no "DIA" display, I also tried a number of different button combinations asides of Defrost and Rest, infact I tried every combination common to rest and everything common to Defrost but still nothing leading me to codes.

I am begining to suspect that the rest function could be what is causing the battery to drain as rest is susposed to allow you to recieve heat from the engine compartment for a limited period with the keys removed, it appears to me that because of the continuios clicking (activation) of the Mono heat valve under the hood that the rest function is running all of the time and refusing to go off, hence not allowing me to run the self Diagnostic.

I know that if I take the car in to the Mercedes dealership they will recommend replacing the in dash operating unit, I will do some asking around for recommendations for specialized auto electronic shops that might be willing to repair the unit.

Thanks for all of your effort on this one.
Colin