Westy,
Given the size of the courtyard and the height of the obstructions around it, I'm not surprised that approval has been turned down by the German CAA. The Bo105 may be a twin, but its performance is less than lacklustre when it loses one of them - the remaining one just gets you to the scene of the accident or incident a bit quicker!
Other than that, the normal requirements for approval to operate to an ad-hoc site in the UK is that you require all that has already been mentioned by many of the other respondees, plus you need to get permission from the owner of the site (which I believe the fire department has given in this instance) and inform the local police.
In London, there are pretty rigid controls regarding where you can fly, particularly at night. Maybe there is something similar in the part of Germany you are referring to.
Although many of the guys have said that this is a fairly "normal" sort of flight at night, I think that the site you describe would normally only be considered for an EMS type flight. Fare paying passengers should be offered something a little less risky........