We were walking in the hills of snowdonia the other day and my girlfriend found this in the heather:
I've never seen anything like it. Can anyone identify it? It was the biggest beetle I've ever seen.
The purple colour is a give away, but as in many insect groups there are often very similar species. As pointed out in the comments to this anwer, this is the case for C. violaceus and C. problematicus (differences)
There are websites like observado.org where sightings can be added as a species or for instance "unknown beetle", where they are reviewed (depending on the availability of the volunteering experts) or identified.
I would say that it is a Carabus coriaceus. It might have a size of 30 to 40mm and is very common around Europe.
Unfortunately the English wikipedia article is rather tiny: https://en.wikipedia.org/wiki/Carabus_coriaceus