I have to agree that Tesco's are irresponsible in buying the plants, lots of places only stock the New Guinea Impatiens (a different strain of busy lizzy), which seems immune to the downy mildew that devastates ordinary types. Are you sure you've been sold the ordinary type? Have you still got the reciept / packaging they came in, as I would be very surprised if they are the old type, it's not in Tesco's interest to stock something that may well die (especially on their shelves) if stricken with the disease.
Most people that buy bedding plants are aware of the situation, and steer well clear of the old type that can be affected, as this problem has been around now for at least two years, especially when the new guinea type is available and won't be affected. Could it be that whoever has told you is winding you up?
I would check packaging and wait and see. Your plants might catch the disease if they're the old type, then again they might not. If NG impatiens, they won't.