Ring has only Two Hallmarks- What Does this Mean?

by Laura
(Leeds, UK)

I was recently bought an engagement ring from a reputable antiques dealers and was told that the ring was 18ct white gold with a 0.66ct diamond. (the diamond is a european old cut if this helps with placing it ain a time period...)

I have examined the ring and the only hallmarks it has are a 750 mark and the rose for the Sheffield (uk) Assay Office. There is no makers/sponsors mark and no date mark or crown/orb or similar to indicate whether it is actually gold or platinum or whatever.

What does this mean?

Have we been sold a fake? I would be terribly upset if it is...
Thanks