Absolutely no idea - I don't have the Mail... ... .... but if you put the clues up as well I (and others) might have a chance of helping!
BTW, if these definitions match the clue for 18d then niello could well be right:
1. a black metallic substance, consisting of silver, copper, lead, and sulfur, with which an incised design or ground is filled to produce an ornamental effect on metal.
2. ornamental work so produced.
3. a specimen of such work.
4. a print on paper made from an incised metal plate before the application of niello and associated esp. with 15th-century Italy.