I play and collect National resonator guitars, which were made out of Nickle plated German Silver during the Depression.
I've used SimiChrome, but also a product called MAAS Polishing Creme from MAAS International. I think the MAAS creme is less abrasive for metals. I beleive that product is available in Europe.

You might also check out metal cleaners for Porsche, as I remember they sell a factory product very much like Simichrome for plated metal cleaning and restoration.