Hi, From your description it sounds like British hallmarks.
This stamp indicates that your item was hallmarked sometime between 1975 and 1998. (750 was not used before 1975 in London I think, before then it was 18)
It is 18 carat gold. (750 again) Your item was assayed in London. (the leopard- it looks like a cat) Your item was hallmarked in 1987 (letter N)
there's a hallmark wizard on this site in the Jewelry Resources, if you want to check the shape of your N. It should be Italic.
Rating
Cats and Letter Markings on Jewelry by: Anonymous
Good Morning,
no image, or any glimmer of understanding about the age of your ring?
Marks, resembling UK marks are known as pseudomarks. Without an image though, it's a bit like playing tombola.
The face of a CAT (big cat -actually a Leopard) is the London Town mark, from 1822. CAT could also be letter. The N sounds like a year stamp.
The CROWN (inside a square) followed by 18 (inside a square) is found on UK - on 18ct GOLD - ca 1798 - 1974.
One possibility that you might like to investigate; British marking within the European Union Hallmarking System. The crown I believe is a voluntary mark.
