I think I've got to the bottom of this.  At least, what I've found seems rational and, as often is the case, so blindingly obvious that I kicked myself for not thinking of this before.

The fields I was having this problem with were always ambiguously-named fields - two or more tables in the SELECT each with the same field name.  So I was relying on the tablename prefix to identify which field was which

SELECT stockrecord.stockcode
  , supplierstockrecord.stockcode

This seemed to work in the field properties tab, and in the Update SQL tab too, but this is where I wrongly assumed that I could do this for IB_Controls too.  It certainly works for the displaying of fields, but not if updating them.

The solution which seems to work (until I prove otherwise) is to alias the non-updateable fields using AS.  So if I'm adding records to the supplierstockcode table I would say:-

SELECT stockrecord.stockcode AS srstockcode
  , supplierstockrecord.stockcode

Hopefully that's resolved it, but if not I will be back.