You're welcome Ian, thanks for posting back.
I'm sure Gary's student appreciates it as much as I do.

Regards
Martin


"Ian" wrote in message
...

Many thanks to Martin for elegant solutions that I should have been able
to
think of but didn't. Thank you also to Gary's student whose reply has had
me
scratching my head for some while until the penny eventually dropped. I
am
grateful to you both!