I have checked the 31 character length option but still find after burning that the disc only displays 16 characters whereas other programs display the full 31 characters.

Of course does NBR support up to 32 characters for DVD-Viedeo discs. You have simply switch to another view mode. In this case you should select 'View->ISO Level 1' to be able to enter/view more than 16 characters for volume name. By default Joliet/UDF names are displayed, where the volume name length is limited to 16 characters.

Nero does not write long volume labels right. Ive also had this problem.
DVD Decrypter has no trouble making a 32-character UDF volume label,
but Nero limits you to 15 characters. "THE_THIRTEENTH_FLOOR_16x9"
becomes "THE_THIRTEENTH_". If you want a long volume label to be
written correctly, you must use Nero Image Writer and then change
the label with DVD Decrypter. In short, Nero sucks.

This is unfortunately a long-standing Nero bug. Nero 5 and early versions of Nero 6 created DVD video discs with the correct volume label length. Then Ahead broke it and now they pigheadedly insist 15 characters is correct, refusing to fix the problem. Why? Who knows, maybe they decided to adopt Microsoft's old habit of denying bugs. I'm really ticked off, I paid for this software and they could care less about sorting this out.

They're quite obviously wrong about the volume label length - I have piles of original DVDs with longer names and every other piece of burning software I have tried allows labels longer than 15 characters. Unless they can produce the document on which they based this, and explain why everyone else has longer labels, and why DVD players are quite happy with them, I am just going to assume they messed up and take my business elsewhere. In future my company will not be buying Ahead products.

Won't do you any good - it shows the correct label length in the GUI, but burns the disc with a truncated label.

UDF is not restricted to 16 characters. Not according to the specification anyway.