you sound like you are really enjoying investing in antique jewelry! The ring appears to be fully hallmarked, however, the image you submitted is upside-down! Francis, can you see a DUTY MARK? The profile of King George IV was used between 1822 - 1833. I have played around with the image as best I can, and I can see clearly two marks only.
The British Crown alongside the gold purity mark - 18. This mark is found on UK 18ct GOLD from 1798 - 1974. So the only thing we can tell for CERTAIN (because we can't see the other marks) is that the ring was made before 1974.
The date letter, the maker and the assay office mark is what is needed to pin-point an exact year. Can you see these marks? If you can, you will be able to date this ring with the information found in Researching British Hallmarks.
Unfortunately, we can't see the settings of these stones either. The settings and the diamond cuts will provide dating clues, Francis, FLUSH settings are a modern cut (usually reserved for diamonds only) in comparison to say the older diamond cuts. Below is an image of a FLUSH Setting.
Flush Setting
The cut of the diamond is very important. Most diamonds today are brilliant cut and have 58 facets. Most diamonds in the Victorian period were cut in what's known as old mine-cut.
