Melt butter over medium-low heat in a large heavy saucepan.
Add the onion, celery, thyme, salt and pepper. Saute until onion is soft, stirring occasionally.
Drain liquid from can of clams into a measuring cup. Add enough water to make 1 cup of liquid.
Add the clams, the reconstituted can liquid, clam juice, stock cube and potatoes to the pot with the sauteed mixture. Turn heat to high and cook until liquid begins to boil. Reduce heat to low, cover and simmer about 15 minutes, or until potatoes are tender.
Mix the thickening ingredients until smooth. Gradually stir into the hot soup mixture in the pot--this will be VERY thick!! Cook, stirring constantly, until mixture bubbles.
Add the cream, stirring while adding, until soup is of the desired consistency. You can use more or less cream as desired. More if you like thinner soup, less if you like a really thick soup. The one cup of cream produces a thick-ish soup.