Put onion, ham, celery, chicken stock, water and chicken in 8 litre soup/stock pot. With the pot partially covered bring to the boil, lower heat and simmer for about 15 to 20 minutes or until chicken is cooked through. Remove chicken and set aside.
Add sweetcorn, potatoes, milk, thyme and pepper to pot, salt is optional. Simmer, uncovered until potatoes are tender.
Cut chicken into 1cm pieces and return to pot.
Thicken soup with cornflour or flour, adding in 2 tablespoon increments and stirring. Once soup has reached desired thickness let simmer 5 minutes and then serve.