Well - thank you to all who replied. Plant Mad Nige offered a tentative ID as an Angle Shades Moth - but here's the pic of their caterpillars from the RHS's advice sheet on the subject:
Not really very similar to my little critter:
But enter the RHS's Principal Entomologist (no less), A J Halstead, who suggests my little friend is a rather more humbly-named Cabbage Moth - aka Mamestra brassicae. I looked this up on UK Moths - and sure enough:
I think we have our culprit. Sadly he is a culprit, too - the moth is described as a "notorious pest" on UK Moth's info page on the subject, scoffing not only all members of the brassica family but clearly pelargoniums too.
Still - I love solving a good mystery. Now I have to go murder a lot of rather pretty caterpillars...