Alex,
I used the data card and you should be able to get one if you have documents proving you own the vehicle e.g. the nova certificate or similar.
Try Sandown Mercedes in Poole as they are a specialist Mercedes dealership for supporting classics. Look them up on the internet.
I did not have any other documentation stating the first registration. I didn't even know the country of first registration, let alone the date.
I got a copy of the data card through the classic centre in the USA, you could try these too.
Regards
Peter