In the end I have settled on approaching the problem the other way
round by detecting when the parent message is opened and writing into
my own property on the attached message the data I require from the
parent message such that when the attached message is opened I have
what I need.

I did not think before that I could do this as when trying to do it
from the Object Model events, attempts to write to the attachment
table resulted in access errors.

However, it does seem to work from the Extension OnRead() event.

Thanks for your help.