I have a diamond ring but can't make out any of the hallmarks. There is a number 12 on the band , does this have any relevance to the diamonds.

The band would not have any reference to the diamonds. The band would only give you the information on what grade of gold it is a # with a K after it. If you need an appraisal, go to a jeweller.

Tick

If you trying to figure out whether the diamond is genuine or not, then its better to get it checked by the jeweler or gemstone expert, since the band wouldn't give you any real clue about it!