Can I wear a short dress to a black tie event??

[KoalaDownUnder]

Dress is black, cocktail-y, designer. Was expensive and looks it. Unfortunately, it is not floor length.

Can I wear it with heels, sparkly jewellery & clutch etc, to an evening event with dress code 'black tie'? Or is full-length mandatory?

Yes you can normally wear cocktail dresses to black tie. Unless it's an actual "ball".

There you go, if you can't trust Debretts then I give up.