blight

If the plants were sprayed with Dithane as a precaution then maybe the infection is present in the plant but doesn't destroy the fruit.

Just had a quick google and found this on the RHS site:

http://www.rhs.org.uk/Advice/profiles0803/tomato_blight.asp

"The fungus can be seed borne, so do not save seed from infected fruit"

 

Thats a real puzzle Brian - it would be a shame to give up though, maybe next year grow just a couple of toms and use the extra space for the stuff Kristen said in post #2.