Empty the prawns into a sieve resting over the sink to drain off any liquid. Tip into a bowl. Squeeze over the lemon juice and season with salt and pepper. Add the mayonnaise, ketchup and salad cream and stir gently to mix.
Shred the lettuce by carefully cutting into thin strips with a sharp knife. Cut the rolls through the middle and open them up. You don’t need any butter as there’s quite a lot of fat in the mayonnaise. Divide the lettuce between the rolls and spoon the prawn mixture on top, then sandwich the rolls together. Put each roll into a paper muffin case (this looks pretty and helps catch any of the filling if it falls out). Chill in a sealable container or a tin until needed.