Bake potatoes 1 hour in a 200 C / Gas 6 oven. Scoop out the inside of the potatoes and set aside. Reserve the skins for another recipe or discard.
Melt the butter in a large frying pan over medium low heat. Stir in the flour to make a roux. Cook about 1 minute, stirring constantly. Gradually pour in the milk while stirring until all the milk has been added. Bring heat to medium and keep stirring until the soup mixture starts to get thick.
Add the potatoes, spring onions, salt, ground black pepper, bacon and cheese. Stir well and continue to heat for about 15 minutes, allowing the flavours to blend. Stirring well, mix in the soured cream until well blended with the soup. Serve immediately.