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Your sawfly larva looks like a good candidate for the suggested species. A better one I feel than my example which is 3rd in the carousel at this stage. The dark spotting on your one seems more consistent with C. septentrionalis.
Hopefully more iSpotters will comment on it, sawfly latvae seem to be tricky to say the least!
Lat/Lng: 54.12851, -0.78513
OS grid ref: SE794709