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Wonderful, yes these things would remind you of an aquatic leather jacket. I should have mentioned above their vicious little robust heads with mouthparts.
Thank mbf45, I think you could have put me on the right track, crane flies abound where these were found and if anything, 2.5cm was a conservative length for the larvae.
Another pic added showing 4 pairs of rear prolegs and the correct forked gill structure at the rear end, its just about visible.
Lat/Lng: 54.994, -6.1057
OS grid ref: NW374743
Small upland lake.