So, realising that I'm not alone in not knowing how to do this I summoned up the courage to look through *all* the functions, no matter how oddly titled, and I found Opaque which does the business (arguably I shd have done this before - but I needed to have my back to the wall...).

For the record, this does it:

use Image::Magick;

my $image = Image::Magick->new;
my $file = $image->Read('old.gif'); 
$image->Opaque(color => '#000000', fill => '#0000ff'); 
$image->Write('new.gif');
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§ George Sherston