[mandible (OP)]

Red Admiral caterpillar seen on North York Moors Friday 13th March. Looked healthy and very mobile. Should it around this time of year? Do they hibernate? What will happen to it this Summer?

[Mazurka]

Yes.  Although most Red Admirals migrate, they can also hibernate and their eggs can survive the UK winter.
By the summer it will have pupated and then may either fly south or hibernate...

[mandible (OP)]

Thanks Maz.