Melt 25g/1oz of the butter in a large pan over a medium heat. Add the shallots, leeks, celeriac and carrots and a pinch of salt and gently fry for 4-5 minutes, until softened, but not coloured.
Add the white wine and bring to the boil. Cook to reduce the liquid completely, then add the water and chicken stock and bring back to the boil.
Simmer for 10-12 minutes, then add the courgettes, French beans, peas and cabbage and simmer for a further five minutes.
Add the cream and whisk in the remaining butter.
Add the tomatoes and the herbs. Stir well, and season, to taste, with salt and freshly ground black pepper and the caster sugar.
To serve, pour into warmed bowls.
