Thanks Salmo,I have researched Dart Moth caterpillars (Cutworms) and am satisfied that is the correct identification.
However, I can't agree with your statement that "they are not a pest".
Apart from the damage they are doing to my leeks I'm sure that it is the cutworm that's responsible for chewing off the stalks of my turnips and earlier having chewed through the stems of some sweetcorn just above soil level.
I went on safari today and removed 46 of them, at various stages of growth, from between the leaves of leeks. Anyone watching would probably have thought that I had taken leave of my senses; fishing them out from the depths with a pair of chopsticks.
They were deposited on the bird table and had all disappeared within an hour.
I also note that they come in a variety of guises but the common feature is the rows of dark dashes along both sides of the back.
Here's some of today's 'crop';
One similar to yesterday:

A selection of the others:

I hereby officially declare war on both cutworms and slugs.
Birds please note that WirralWally's patch is the place to be for a first class spread.