On the advice of @Lee, I have got some 15mm rear wheel spacers. I fitted them this afternoon, and I am really pleased with the change of look. The car looks much meaner now
. I don't know if I was imagining things, but when I sat in the seat after fitting, the car actually felt lower and wider. I will put some pictures up, but the light was poor and doesn't do the spacers justice.
Before
After
Before
After