A section of code that worked for me in July only partly works now. One 
issue that I've noticed is that

\group_begin:
...
\DeclareDocumentCommand \test ...
...
\group_end:

will not in fact create a document command \test -- I get an "Undefined 
control sequence" message when I try to use it in the wider document. 
The declaration must be outside the group commands -- at least that is 
what my experiments tell me. (But \test is recognised within the group 
commands.) Is that correct? And is it new? -- I don't recall meeting 
this issue back in July.

The xparse documentation doesn't seem to mention this issue and perhaps 
should. I can understand the documenters assuming that a document-wide 
command would only be defined at the highest level, but in my context 
(working with instant preview in LyX) it was quite natural to define it 
within the \group_begin:, \group_end: commands of another funtion. I've 
changed that now which has resolved at least some of the problem.

Andrew