Hi I have a diamond ring with a gold band with a platinum setting and round shaped diamond, it is marked with PLAT 18C and a T with a small line underneath it, I'm sure that the PLAT and 18c means platinum and 18 carot gold but I would like to know what the T with a line under it means, is it a hallmark or another mark please tell me.

Thanks

Are you sure it is a T with a line under it? It could be an anchor which is its hallmark and that would mean it was hallmarked in birmingham, If not then it is the makers mark. Every manufactuere has to have their own hallmark or they can't have it asseyd for authenticity. Hope that helps x