Thanks guys:
gg- had already tried that - no luck.
MB - same result after that
Puzzle is that I went to try and un/reinstall SQL Server Express and it is not there! Possibly a clue

,but why the update prompt? Do I have a 'dirty' registry perhaps?
EDIT: Hot on its heels - Cannot install Security Update for SQL Server 2005 Service Pack 2 (KB970895)" Strange.
EDIT 2: Google indicates I am not alone.