...this week I will be learning more than I ever wanted to know about Tomato Blight and how to salvage something once it sinks its nasty little fungal wotsits into your crop...
I suspect it's a combination of the wet summer and the relocation due to building work that's caused it. And it looks like all but one of my plants has some degree of infection. Bleah.
Phase I (as recommended by various books) - severe pruning and safe disposal of the affected bits (i.e. do not compost)
Phase II is to get something to spray 'em with to slow any remaining infection and increase the chances of getting a few intact tomatoes...