Oh darn, that's so disappointing just as you get close to cropping! Not sure about milk in the soil, the fats in it may coat the roots & prevent nutrient uptake?
From what I know about growing toms it's not so much a calcium deficiency as a lack of calcium in the right place in the plant.
Give them a big watering to flush the pots. If you've got any good, live compost top them up & water in. Then remove all the leaves except for the top 25%

this is what the RHS says
https://www.rhs.org.uk/advice/profile?PID=395

basically not enough flow of water. usually plenty of calcium in the compost just not enough water to allow for its uptake. don't add milk as the calcium is in the wrong format.
we had the odd one but the water system was increased and the problem isn't happening now.

I have been watering mainly from the bottom and they have been using at least 1 litre each every day from the bottom plus what they get with tomato feed from the top two or three times a week. So should I discontinue that and water from the top?
Darren why do you say remove three quarters of the leaves?

It's a trade off. The older, lower leaves become so inefficient at taking in sunlight & converting it to good stuff that on a cost/benefit analysis better air circulation & getting the nutrients to the toms by removing all the leaves is better.
If you've got blossom end rot on a plant that's only meant to give you toms, why bother about leaves? Send the nutrients to where you want them?

Removing some leaves may increase the root pressure in the plant at night, which is what pushes calcium ions into the fruit. The trade off is that the fruit needs at least some healthy leaves to provide the calories it needs to grow.
Here in the desert, I find that 99% of the cases of blossom end rot are due to insufficient, shallow and frequent, or irregular watering. Actual calcium deficiency only shows up in a few cases of growing in pots with synthetic compost and softened or RO water, or in mismanaged hydroponic systems.

I like you have been suffering from 'blossom end rot'. I read the egg shell piece but decided to give them a dose of granular garden lime. The product says that it also contains a little magnesium. The recent ripening fruit seems to be OK but I wonder if the treatment would have worked in just a week.
So I think that the calcium treatment should have been included in the original soil preparation. On the watering front, I have been watering regularly morning and evening in the hot weather. My only problem was my cherry tomatoes mostly split. I did wonder if some varieties were more susceptible than others to blossom end rot.

Thank you very much everybody. (Knew you would turn up trumps!)Most of the advice adds up to water more regularly but I have never let the compost dry out and they have at least a litre each every day and twice if the weather has been very hot. I have watered mostly from the base in order not to flush out fertilizer components from the compost - the plants stand in a tray which I fill with water every morning , and again in the evening if necessary. When they have tomato feed that is watered in from the top. I will try top watering more and remove some leaves.
I also put a sprinkle of volcanic rock dust round them as it has had such a dramatic effect on my brassicas.

Re removing leaves, the bottom leaves are big but the top ones are smaller and rather curled. If I remove some bottom ones do you think there well be enough left of the smaller ones to do their job?
I will let you know what happens.
S

I read a USDA report recently, haven't managed to find it again to provide a link, that the oldest lower leaves can be as low as 15% efficiency. I'll look for the piece later when I've got time. It was by USDA Tellus

No, can't find that research anywhere.
Can you check the pH of the soil? 6.5 is ideal for toms.
A bit out there but could you spray with a crushed antacid tablet in a litre of water & a drip of washing up liquid? Antacid tablets are high in calcium carbonate

Jimmy I have run out of garden lime, hence trying a little crushed chalk (didn't do the base of the metal teapot or the wooden chopping board a lot of good but they'll survive...)

I once read somewhere that eggshells have to be ground down to a very fine powder and thrown into the hole in which the tomato is to be grown.
Sadly I forgot to do this and ended up with the dreaded black bottoms. But I’m now saving eggshells and will definitely do it next year.

In the ground, a proper watering for a tomato plant involves around 30 liters at a time. In the desert, that should happen twice a week, though of course less often in cooler, more humid climates.
In pots, water enough to saturate the compost, plus a little to drain out. If that is less than 2 liters, though, either the soil isn't actually getting saturated, or the plant is in too small a pot.

Its saturated Tug. What size pot should I have used?
Anyway now I have fungal leaf spot (todays question) so maybe all hope is lot anyway.

Hate to say it but the fungus could also be exacerbated by insufficient water at the roots

I usually recommend a 15 to 20 liter pot for one plant.

Darren the compost is really wet.

Tug my pots are only half that but I haven't had problems before. But where you are keeping anything well watered must be quite a challenge.

Happily the kind neighbour who gave me the plants has promised to give me some of hers when they ripen! There are also three fruits unaffected and still lots of flowers so haven't given up yet..

During the first very hot spells last month we got a few with BER. I increased the watering and the new fruit are unblemished. So don't give up on them.

I haven 't totally given up but am getting a bit despondent. I three good unblemished ones but they don't seem to be growing any bigger. I'm wondering if Origon Spring need different treatment from the British varieties.