Place the prawn shells and heads in a pot and add the stock, bay leaves, peppercorns, salt, lemon peel and garlic. Bring up to a boil and allow to simmer for 30 minutes, covered until the stock is well infused by the shells.
In a separate pot, melt a little butter and fry the onion until soft and translucent. Add the garlic and fry for a few seconds before adding the smoked paprika, tomato paste and sugar.
Allow to fry for 30 seconds then pour in the prawn stock with the shells and heads.
Allow to simmer for 20 minutes then remove from the heat and allow to cool to room temperature.
When the stock has cooled down, blend, shells and all until the shells are broken up completely. Pass the mixture through a sieve into a bowl and then through a sieve lined with muslin once more into a clean pot. This is to make sure there is no grit or sediment in the bisque
Place the pot over a low heat and allow to come back up to heat. Pour in the cream. If the soup seems too thin at this stage (which it probably will be), whisk in the flour paste and allow to simmer gently for 10 minutes until the soup has thickened slightly. You don't want it to be too thick, it needs to be very 'soupy' still but also not watery.
Season to taste and serve with a sprinkling of chopped parsley.