Could you try if it works through the console?

Actually, had another go looking at this and solved it by using imagick instead of ext_imagick.  I don't know what the difference is, but it works, to give

Graphics Library: ImageMagick 6.5.4-7

// Library used for image resizing. Value could be 'auto', 'imagick',
// 'ext_imagick' or 'gd'. If value is 'auto', library will be choosen in this
// order. If choosen library is not available, another one will be picked up.
$conf['graphics_library'] = 'imagick';

// If library used is external installation of ImageMagick ('ext_imagick'),
// you can define imagemagick directory.
//$conf['ext_imagick_dir'] = '/usr/bin/';

Problem solved!