It must be related to inkscape, although it's not listed in the output from 'apt-cache rdepends inkscape' :
inkscape
Reverse Depends:
inkscape:i386
zoomer
ubuntustudio-video
ubuntustudio-graphics
ubuntu-edu-tertiary
ubuntu-edu-secondary
sozi
python3-matplotlib
python-matplotlib
phatch
openshot
openclipart2-svg
openclipart-svg
open-font-design-toolkit
multimedia-graphics
lyx
latex-make
kdesdk-misc
kamerka
ink-generator
gnome
I know next to nothing about package management, perhaps one of those items in turn depends upon imagemagick?
In any event, it must have been installed when I used apt-get to install inkscape (I've just opened synaptic and got the first run welcome dialog so I know I haven't inadvertently marked it for install there). imagemagick-common is also installed and when I mark it for uninstall it tells me it wants to uninstall inkscape so I'll leave that alone for now
If inkscape falls over I'll re-install imagemagick itself.
Now, I just checked a Linux Mint 13 (MATE) machine at work that has inkscape installed (the same version as the one on LL), and it
does have imagemagick installed... I've never noticed it though because it doesn't show up in the menu.
Bottom line, problem solved and thanks for the help!
Paul