Custom number format disappears

[Anthony]

Thanks for the rapid reply.

I think the problem is more complex than this. All my
historic files HAD this custom format when they were
created and on many subsequent edits. It is just that it
has recently disappeared in all of them (and the custom
format box is filled with lots of custom formats I have
never used and would never want to use!).

I do have the custom format set in my Book.xlt and
Sheet.xlt files and have had for many years.

Any ideas how this could happen? Perhaps something has
become corrupted but what could it be?

Thanks

Anthony