Found on the High Moor during a Hoots volunteer day.
Photographed by Wendy English.
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Can only be an Emperor as you suggest. Have seen them fully black at 1cm, 2cm and black and yellow at about 3cm, I can't recall seeing it at exactly the stage you have photographed- tiny bits of green showing before any yellow, but the green/yellow colouration does appear first at the periphery of those nodules and progress from there throughout the caterpillar.
Thanks heaps Cathal- great to have id confirmed, Tanya
Fylingdales Moor, North York Moors
Lat/Lng: 54.3764, -0.6197
OS grid ref: SE897987