Liquidise the two tins of tomato with the 250 ml water until quite smooth and strain through a coarse sieve, rubbing with a spoon to retain the tomato pips.
Place the butter, garlic, leek and celery into a saucepan and sauté very gently, stirring for two minutes, without browning.
Add the strained tomato, additional water, stock powder, herbs and sugar and bring to boil.
Simmer gently for 10 minutes.
Mix together the flour and cream in a small bowl.
Stir a little of the hot soup into the mixture, stir well and return to the soup.
Add the salt and pepper and cook over moderate heat for about 5 minutes to thicken the soup.
Do not allow to boil rapidly - the soup may curdle.
For a smooth soup, blend a few seconds with a hand-held blender.
Press the cheese onto the thin bread slices, sprinkle with paprika and place onto a baking tray.
Just before serving, grill the toast until nicely browned.
Adjust the seasoning, ladle the soup into warmed bowls and float a cheese toast on top.
Garnish with chopped spring onion.