Seen today at Rainham Marshes, Essex. About 3cms long.
Looks a lot like a bibionid larva, though 3 cms is perhaps too long for one. See -
Many thanks for your reply. However, "my caterpillar" had "spikes" on its "spikes" which the bibionid larva does not appear to have. Also, the "caterpillar" has a black dot on each body segment which, again, are absent from the larva.
Might the larva develop these as it grows?
Definitely not a fly larva - looks more like a moth or butterfly caterpillar to me but we'll need a specialist to tell us what it could be
I'm far from being a caterpillarist (or a lepidopterist), but to me it looks like a larva of a Nymphalidae, who typically have such "spiky spikes" (and at least some early instars of some species are rather dull in their colouration).