You're right GK, It is an absolute joy having a summer full of butterflies and moths, but......it also means that there is a dizzying array of weird and wonderful caterpillars. Generally they come under the Hawk Moth family, although not always. The Peacock butterfly is black with coloured bits. Was it large or small - smooth or hairy?
Until someone comes up with the definitive answer have a browse on this wonderful site devoted to UK caterpillars.
http://www.whatsthiscaterpillar.co.uk/