Davidian, the other two members who have helped you are perfectly correct, but there is one more "gotcha" with just quoting a catalogue number.
If someone tells you they have SG 230a you naturally need to know the country first, because many different countries will have a 230a. The further assumption is that it is 230a in the current edition of the catalogue, because over time the catalogue editors do renumber things and change numbers around, even though they try not to cause too much confusion when they do it. So if you want to nail it down properly you will need to know which country for the SG 230a, and from which year or edition of the catalogue they were quoting. Sorry it all sounds too confusing ... in practice it is so much easier because just grab any edition of a Stanley Gibbons catalogue that covers your country and it will do.
If it were Australia, for instance, they did renumber that part of the catalogue (many years ago), and SG230a is a 4d. green koala stamp and SG230b is a 6d. purple-brown kookaburra.