Steam the salmon and the celery in a steamer until the salmon is no longer pink in the centre and flakes easily with a fork, and the celery is tender. Remove and flake the salmon being careful to remove any bones. Allow to cool and then finely chop the celery and place both in a liquidiser.
Add the cream, mascarpone, lemon juice and horseradish sauce and blitz thoroughly. Slowly add the milk until a stiff consistency is reached. Add salt and pepper to taste. Place in a bowl and cover and place in the fridge.
Prepare the carrots by chopping into small cubes before adding to 500ml boiling salted water. Simmer gently until soft.
Place the carrots and a little of the cooking water into a liquidiser and blend until smooth, slowly adding the rest of the water. Place in a saucepan and allow to cool.
Prepare a large flat pan by lining with cling film.
When cool sprinkle the gelatine over the liquified carrot and then gently reheat until the gelatine has dissolved.
Pour into the lined pan and spread out evenly. Make sure the pan is level and allow to cool.
Meanwhille form the pate into sausage shapes about about 4 to 5 inches long and 1 inch in diameter, and roll tightly in cling film. By now the carrot jelly should have cooled and set. Place this and the pate in the fridge and leave overnight.
Now for the complicated bit: The next day remove the carrot jelly from the fridge. Cut a sheet of cling film and cover a chopping board large enough to cover the pan. Place over the pan and then turn upside down. The jelly should drop easily out of the pan and the original cling film can be peeled off.
Cut strips of the jelly to the same width as the length of the pate 'sausages'.
Unwrap the pate and place gently on the jelly. Using the underlying cling film gently roll the carrot jelly around the pate. Place back into the fridge until required.
For the balsamic reduction: Place the vinegar into a small pan and heat. Boil until reduced by about half its volume then slowly add the sugar, tasting as you go. Allow to cool before serving.
If available use the leafy tops of the celery as a garnish. Even when chilled the cannelloni are very delicate and need careful handling. I found it best to use a scraper or a palette knife.