I assume you're practicing crop rotation and this occurs with more than one variety of tomato? There are quite a few different problems that could occur with tomato leaves, but it kinda sounds like bacterial canker to me. If so, apparently it is carried with seeds so if you're saving seed or using the same seed then that could be one of the reasons you're always having the problem.

Here's a good link about tomato diseases

Here is a link that might be useful: bacterial canker & others

No I don't save seed I buy my seeds from one of 3 reputable seed catalogues, so I don't think it is bacterial canker. from the website you put up it sounds more like leaf spot, but I'm not sure of that either. It is worse with cool damp weather. It does happen with every variety and not just one. Whatever it is is in the soil because if I grow them in pots they are ok.

Hey oilpainter. I'm not an expert, but it seems to me you may have blight problems. I'm pretty sure the same blight can affect strawberries and tomatoes. Do the stems of your tomatoes get any black streaks on them? I had blight on my tomatoes in B.C. and if I recall that was one of the main indicators. Blight stays in the soil as far as I remember, and that would explain why you don't have a problem when you plant in pots. Maybe the internet would have pictures you could compare with?