The large number recorded suggest that these moths may not have been migrants helped northwards across the English Channel by south to south easterly winds but instead may be part of a resident population here in Cornwall. Time and further recording will hopefully confirm.
Insight into Portland Ribbon Wave (Idaea degeneraria): The only known UK resident population of Portland Ribbon Waves is on the Isle of Portland on the south coast of Dorset. Annually there are a few records along the south coast and these are thought to be migrants from the continent.
Reference: Field Guide to the Moths of Great Britain and Ireland by Waring,Townsend and Lewington.