Place potatoes in a large saucepan and cover with water. Cover the saucepan and place over medium high heat then boil the potatoes until tender. Drain reserving 125ml cooking liquid.
Roughly mash the potatoes in the saucepan, leaving some large chunks of potato for a 'rustic' consistency.
Meanwhile; fry the bacon until crisp then remove to kitchen paper to drain.
In the same frying pan saute onion, celery and carrots in bacon dripping until tender. Add to potatoes. Stir in reserved cooking liquid, chicken stock, milk, sour cream, salt and pepper. Cook over low heat for 10 minutes or until heated through; do not boil.