I was not alone . . .

. . . in thinking this was the larval stage of a ruby tiger moth.

Whilst researching its ID I came across someone who had also wrongly assumed (at first) that the subject was a ruby tiger moth but . . .

The ruby tiger moth has a black face and this one definitely had an orange/chestnut face (though it is not visible in this view).

I was then left to decide between a couple of ermine moth larvae.

The white ermine moth caterpillar has a pale/white dorsal stripe and lateral light/white spots.

This one lacks the dorsal stripe and the lateral feature is not spots but an almost continuous pale line.

I therefore conclude that this is the larva / caterpillar of a buff ermine moth Spilarctia luteum.
It feeds on a variety of herbaceous plants and in this case was feeding on Aquilegia (columbine). I found it because as I was weeding my mother's garden (Yes! In the rain again!) I noticed some frass on a piece of pale flint and looked above for the culprit - and here it is (chance favours the prepared mind - Louis Pasteur).

If you think it is something else entirely, then please, let me know.