Poor little chap - heat and no potash...

Never seen this before, but it seems like a perfect example of too much heat (outside), and no grub for a week or so!

Unless it's something nastier...

Urgh nasty 
Is it all the fruit on that plant?  It may be a virus - cull the plant, or just one odd tomato - throw that 1 in the bin, and keep an eye on the rest ….

Same thought, Jay

If I saw that, without any comments/history attached, I would be thinking they had been attacked by an insect, snails, or something else.  We have tomato fruitworm which digs holes into the fruit, but not quite like that, usually an entrance and exit hole, so they can live inside the fruit, eat from the inside out.  Maybe stinkbug damage?  The more I look at it, the more I think the brown marmorated stink bug, or BMSB (Halyomorpha halys) could be the culprit.  They have recently become a problem in the UK (they are an exotic pest). Do a computer search for VCE Publications / 2902 / 2902-1100 (VCE is Virginia Cooperative Extension), look at the photo near the very bottom.  My son is an entomology graduate student, is studying BMSB, they are a pest on many crops.

I know you binned them, but did you see any signs of insect "frass" (small pelleted insect poo, usually dark) below the fruit, on the leaves?  I look for that when I am inspecting plants for insects, especially caterpillars.

Plants are usually pretty capable of translocating potash from one place to another, or extracting more from soil, I don't thing skipping the potash for 2 or 3 weeks caused this, certainly not your fault!  I would add that heat doesn't seem right.  I have had 38 C temperatures at least once this summer, greater than 33 C many times, my tomatoes haven't faltered.

[SP added comments about stinkbugs and temperature]

Growster, I will tell my son you were able to make use of what he has learned (and what his old dad has learned from him)!  BMSB should be controllable with many of the common controls you might already have on hand for your vegetables, but I will see if he has specific suggestions.

I've been around all the outside plants today, and while the 'Shirley' are ripening in different degrees, the others are just fine (except a few Mountain Magic), so it's a watching brief at the moment!

As Al78 (Good year that), says, the temperatures have been awful these last few days, and now Southern Water want everyone to stop watering stuff!

I'm also peering at the peppers nearby, in the same bed, as they might just get some idea of a revolt, so we'll watch what happens!