Identifying caterpillars shouldn't be too difficult should it -especially the big flashy ones.
When my daughter found this one strolling across a grassy footpath on the Bamburgh cliff top I took a quick photo with the plan to sort it out definitively back home.
The first two field guides I tried were inconclusive, with none of the illustrations matching this one exactly or with illustrations so small that the detail could not be made out. This included, disappointingly, the Collins Field Guide to Caterpillars of Butterflies and Moths in Britain and Europe. The most helpful has been my old favourite, Margaret Brooks and Charles Knight's 'Complete Guide to British Butterflies' from 1982.
The trouble is that caterpillars can be a bit of a bother. They have to moult to grow, averaging four or sometimes five changes before full size. In some species not only does the look of the caterpillar change significantly at each moult but even the same instars of different individual caterpillars show variations in colour and pattern.
The easy bit is that spiny larvae are most likely to be members of the Nymphalidae family.