I was watering the plants I am growing on in the cold frame today and saw this colourful caterpillar on one of them.
After examining photos online and reading about their habitat and lifecycle I have concluded it is the caterpillar of the Knot Grass Moth. Apparently quite common and widely distributed across the UK. It is very similar to the caterpillar of the Yellow Tail and the Brown Tail moths but all in all I am sticking with the Knot Grass Moth. But happy to be corrected.
The adult moths can be seen between May and July and there is a second brood in August and September which fits with the caterpillar found today. The caterpillar feeds on herbaceous plants so this one turned up trumps in my cold frame.
The adult Knot Grass Moth looks like this (courtesy of UKmoths.org.uk).