First pick the meat out of the crab, reserving all the pieces of shell.
To make the reduction, melt the butter in a pan and cook the carrot, onion and celery in it until beginning to colour. Add the crab shells, tomato, tomato purée and white wine and stir over a high heat for 2 or 3 minutes. Add the water, bring to the boil and simmer for 40 minutes. Strain through a sieve into a secon