This is Ctenochares bicolorus, a rather colourful ichneumonid of the subfamily Ichneumoninae. It is known from large areas of Africa and, rather less frequently, south-western Europe, including Spain. It's a parasitoid of noctuid moth pupae and has been found once in London, in a Malaise trap in Battersea Park, rather strangely.
Absolutely brilliant Gavin. Thank you so much for taking the time to reply. I guess with the lettuce originating from Spain that is the likely source. I shall do a follow up post on our blog - letter from Norfolk - where you will obviously get a mention.