Thanks to HeLen, Adam and Alan for their replies.

For information, tried dropping the constraints and re-adding the PK,
set the sort order to descending and commited. The sort order defaults
back to ascending.

To answer Helen's question 'Why', it's because the table will be
used 'as standard' in descending sort order, not ascending. Therefore
it 'uncomlicates' matters where one has to remember to add an ORDER BY
clause each time the table is used in an application.
The PK is based on a generator, actually a composite field of the
generator and a string, and is used in a grid for selecting the latest
(hence descending sort order) record entries rather than the oldest
(historical) ones.

Will either use a second index or ORDER BY to continue.

Cheers and thanks again!