Purple Heron

I often use the Merlin app to identify bird calls when out on my early morning dog walk. I do have all the latest bird song and bird call libraries for the UK and for Western Europe loaded in the app. This morning I disturbed a large heron that took off in front of me, squawking, and the app recorded it as a Purple Heron. Apparently there are only approx 20 sightings of this bird per year in the UK. I live in Somerset. Has anyone seen this bird?