Extracting Tag Value from a Known Tag Name


I've had a look at this and I'm a little lost as to where to go. I know the tag name who's value I want to get (eg. drn_no01 is 5200-C-0001), but I would like to use this to set a file name for printing and a save-as routine which I have already done.

How would I do this?

Thanks as always,