Rosie, it might be easier if you could ungroup the objects and delete the piece you don't want.
The eraser is flakey IMHO but you could try setting the eraser to rectangle instead of the default circle and set the size to 0.10 or smaller.
Let us know if you've resolved the issue, or post the file and what you don't want in it and someone will try out both options and let you know which works.
LEARN, GROW, CREATE, SHARE
Mac 10.9.5 Yosemite, eCAL 1.506, ECAL2.023, SCAL4Pro, Export Plug-in, Inkscape 48, iPad/scal3