Marc,
The 18-55mm focuses very close (1:3) and I think for what you need it for, extension tubes is a very good avenue to go down as the optical quality with the 18-55 is very good.
Look into is, you may find that spending £40 on ebay for some auto tubes ways up more positives than £350+ on a macro lens...
After an earlier quote i really like the advice and the sound of getting auto tubes to use with my current lens. Has anyone used any with the d3100 which have worked well? Anyone got any spares