Divine the cos lettuce into individual leaves, trimming down the large outside leaves so that they are all roughly the same size – you’ll need 20 in total. Place in a large bowl and cover with cling film. Chill until ready to use. Use the very small centre leaves in a sandwich or for another dish.
Chop the Dublin Bay prawns if large and place in a bowl with the mango, lime rind and segments. Dress with the rapeseed oil, mixing well to combine and season to taste. Cover with cling film and chill until ready to use - you can prepare this up to 4 hours in advance.