Brown patches on leaves are the first signs that blight has affected your crops. The leaves will then curl up and wither. The problem can be that other potato/tomato problems may display similar symptoms. There are photos of blight affected potato and tomato plants here, here here and here. You can also buy a testing device to check for blight from here.
Blight can be a sneaky problem as it can lurk on seemingly perfect fruit or tubers. Tomatoes that appear unaffected can start to rot five days after they have been picked.
It's almost impossible to prevent blight but there are precautions that can be taken. Avoid planting potatoes and tomatoes in the same areas in two consecutive years as spores can be present in the soil. Leaving affected leaves and stems lying around can also increase the chances of passing the disease on. Earthing up can make it less likely that the spores will infect the potato tubers. Another alternative is to grow early varieties of potatoes that will mature before blight conditions prevail or to try some varieties that are supposed to have some degree of blight resistance. If anyone has any experience of varieties that have seemed to weather the blight storm please share your experiences by posting a comment.
A protective fungicide can be used but this needs to be applied before blight is apparent and spraying has to continue every 10 days after the first application.
If your potato plants do succumb to blight then remove and burn the affected foliage - it is possible that the spores may not have reached the potato tubers. After removal of the foliage leave the tubers in the ground for at least two weeks. During this period spores on the surface will hopefully be killed and lifted potatoes may avoid being infected. Once lifted you will need to keep checking the tubers for signs of blight and dispose of any that become mushy or have dark patches on the skin.
Tomatoes grown in greenhouses are afforded a little protection from wind borne spores, however if spores do enter the greenhouse the disease will thrive in the warm humid conditions. It is also extremely likely that tomatoes will be grown in a greenhouse year after year and so any spores remaining in the soil may be more likely to spread the disease from one year to the next. Removing the bottom leaves of tomato plants can help in that plants can be watered directly on the roots thus avoiding any splashing onto the leaves.
After today's rain, I guess for now it is just a matter of sitting tight and keeping everything crossed.
Potato Council quick reference for disease, disorders and pests that affect potatoes.