Confidence: It's likely to be this, but I can't be certain.
Notes: You're right that it's the hortensis group. Not easy from directly above or below. The uniformly pale top, and the smooth upper edge to the dark band, suggest hortensis, and I think I can see a pinkish tinge to the translucent parts of the tentacles, which is strongly suggestive of hortensis. In side view, the wide very pale flanks are .a good feature