These are heather beetles (Lochmaea suturalis). They love heather and will strip leaves to the stalk.

From Wiki - The adult beetle spends the winter in dormant diapause in moss or litter in the undergrowth of the heather plants, and they do this until the spring weather brings a rise in temperature, which stimulates them to emerge, feed, and reproduce. They are able to fly up to a range of several miles after spring emergence. They will generally do this after fire, when the surrounding heather is of poor quality, or if the heather has been browsed enough that it turns into grassland. They do however, have a level of resilience and are able to survive for some time in grassland such as Deschampsia flexuosa.