I seem to remember on small twins you had to add 50ft to an ILS, this was published in the AIP (Probably the place to look for this one). I think it was due to performance issues on light twins which couldn't maintain a great Climb Gradient with one engine out.
In the company I work for we recently changed our procedures so that we no longer Add 50ft to MDA if it is described on the Jeppesen plate as a DA, I think this will come down to the individual approach (Offset etc) on whether its classed as an MDA or DA.
I think there is probably some good info in the Jeppesen text manual on this if you can get your hands on it.