There is a UI in Sønderborg. Southern Jutland - Germany 1864 to 1920. But Danish before 1864. Danish tradition in this area could very well one masters mark. However the mark is UI - I and not J. I and J is though often used randomly.
The UI is Ulrik Johannsen born 1816 citienship 18/11 1843 he Works in Germany and Austria - he is married 1847 and he dies 1912.
His mark is UI or JOHANNSEN or UJ or U.JOHANNSEN I dont know - but he could be a possibility. Even though I would expect the spoon to be a bit older.
Hi Trev, of little or no help to you{!} - I believe the same mark was noted on a pair of Old English pattern sugar tongs and catalogued as 'unascribed but probably colonial circa 1800'. Best wishes, Jo.