Hello Petra,
The reference you have been given is from the General Register Office in London, if you wish to purchase this marriage certificate by ordering online it will cost UK �9-25 = �11-00 and you can pay by credit card go to:
http://www.gro.gov.uk/gro/content/certificates/default.asp
Just follow the links, you need to give some details, if you have a problem contact me again and I will try and advise you further.
GRO Reference: Marriage: Josef Alschner and Auguste Luders, September Quarter, District Pancras, Vol.1b, Page 338. (Luders is the English spelling on the index)
However I must advise you that the information supplied on the British marriage certificates aren't usually as informative as church records were in 1900, just as follows:
Names of the bride & groom, place where the marriage was performed, occupation, age, place of birth, address where they were living at the time of their marriage, father of the groom's name, father of the bride's name and very little else.
You may have all this information anyway. It's a pity they weren't married in one of the German churches in London.
I also have in my personal library on micro fiche indexes to the marriages held at "St Paul's German Reformed Church" in London for 1900 unfortunately they are not listed. Marriage indexes to St George's Lutheran Church in London only go to 1896. See history of German churches in London:
http://www.stgite.org.uk/media/germanchurches.html
My guess they were married privately or perhaps in one of the registry offices,for the reasons you gave.
I have checked the other sites I mentioned previously and was unable to find a full marriage transcription online, only the GRO Index details.
If by chance they were married in a church it would be necessary to know the name of the church and the parish. Perhaps then I could check the LDS (Mormons) "Familysearch" to see if the marriages had been filmed for that parish in London. Any marriages performed privately or by a registry official were unlikely to be filmed by the LDS.
Please let me know if I can help any further. I have purchased many certificates online from the GRO and the service is very good, be sure to give the correct details if you decide to order.
Kindest regards,
Elizabeth