This was about 6cm long and very pale on the underside.
No interactions present.
Thank you inopinatus for the identification and information. The larva was in a damp location close to the loch when first discovered.
As said in subsequent ID, there are 6 possible species and Dytiscus marginalis is just the most common so it's not necessarily that one.
Lat/Lng: 57.58095, -3.6376
OS grid ref: NJ021557