Put the celery and carrots in a saucepan with the stock and simmer for approximately 10 minutes.
Add the potatoes to the stock and simmer for about 15 to 20 minutes more until the potatoes are tender but not breaking up. Strain the stock into a jug.
Meanwhile, fry the onion in a separate pan with the butter or oil until soft but not brown. Then add the bacon after you have cut it into large pieces and fry gently for about 5 to 10 minutes.
Add the flour to the bacon and onion and stir well with a wooden spoon over a low heat until mixed in. Gradually add the stock that you have strained from the vegetables in the other pan, stirring constantly until you have a smooth mixture and all the stock has been added.
Add the vegetables, sage and then the milk. You may need to add a little more water if it is too thick. Bring to simmer gently and cook for about 10 minutes, stirring to stop the chowder sticking on the bottom of the pan until it is hot. Serve in large soup dishes.
You could substitute other vegetables instead of celery like sliced leeks, or frozen broad beans or peas instead of sweetcorn.