Thanks for the reply.
The following macros have all been run with the following results.
Trimall had no affect
ReEnter had no affect
ReEnterF2 does fix it.

I haven't had time as yet to check for the non printing character
issue, but the tips there are appreciated as I am still baffled by the
nature of the failure mode. I'll try and use these tips next week to
confirm that the cells do or do not contain only valid chars.