Have just found that there were other Levi families in King's Lynn in the 1780's -- so these are good candidates for my Moses's family:
http://www.foxearth.org.uk/1783NorfolkChronicle.html has the following transcriptions:
Jonas LEVI, a Jew, son of Solomon LEVI, of Lynn, a youth about the age of 16 years who was attacked and robbed
a sailor, whose name is FOX, was taken up at West-Winch, on suspicion of being the perpetrator of this horrid act
...... FOX, the footpad from Lynn, who robbed and cruelly beat the son of Mr LEVI, silversmith of the same place.
...Robert FOX, convicted of robbing Isaac LEVI of King's Lynn, travelling silversmith, and most dangerously beating him, received sentence of death,
...OX, for robbing and cruelly treating Isaac LEVI, a Jew lad about 16 years of age.
There seems to be some confusion as to whether the victim was called Jonas or Isaac Levi, but he & his father certainly seem to be living openly as Jews in King's Lynn at the time when my Moses was in Diss.
Jonas /Isaac would be about 18 years younger than Moses so could be a brother or a nephew