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I don't think this is Four-dotted Footman. Although in Scotland, I don't think it's ever been recorded in this area. The arrangement of spines would be different and the caterpillar would have a reddish head.
According to Porter (Caterpillars of the British Isles) : "body black, with dense black or dark brown hairs; head shining and black." Even so my identification is tentative.
I have agreed with Cybosia mesomella because that it what it looks like to me,as Michael says Porter says it has a black head .Ukleps website shows a reddish head in later stages and the example I found in Devon was halfway between the two .It is all very confusing .
Shame you didn't try and rear it through, Angusranger, best way to be sure!
Anyway, I've found caterpillars identical to this, which I've put down as late White-ermine or dark Ruby Tiger (no Four-dotted Footman in the area) Did try to rear through but didn't survive.
Four-dotted Footman would also be much smaller in my experience (hand in photo is useful scale) and as I said earlier the hairs/spikes would be different.
Well, it's much more fun when it's not straight forward! Who'd ever want caterpillar ID to be easy?!!
Thank you for all your comments. Very much appreciated. Both white ermine and ruby tiger are common in this area. As you mentioned, possibly best to rear to gain actual I.D. Thanks again.
Lat/Lng: 56.791, -2.7267
OS grid ref: NO557668