Could well be Blossom end rot. This is caused by the plant not being able to take up calcium. Bit hard to treat, but it is generally thought to be a problem from faulty watering. Do a Search as I am sure that there has been a fairly comprehensive discussion about this on the site.

Underwatering, or irregular watering is the usual cause - calcium is transported in water - if the plant goes short of water when fruit is present, it doesn't get enough calcium and you get blossom end rot on the fruits. Once tomato fruit starts growing, watering regularly and deeply should solve the problem - unless you're using really poor soil to grow them in.

I think its uneven temperatures too, very cool last few days now roasting again!

A good thick layer of mulch will help even out the water uptake. Grass clippings or straw work best for me. Also, when you plant your tomatoes, plant deeply so the roots go down far and deep keeping them moist and cool.

Not watering on a regular basis - I use Epsom salts as well sprinkle a bit around each plant I do it once a month when in fruit.