[Lardman]

Not a very good picture as it's a bit bright out there.

I've only seen blossom end rot before on developed fruit - not on developing. This is my black russian , but the same problem can be seen on the roma and black prince, these were ok when I checked them on Monday.

[Aunt Sally]

It's usually caused by inconsistent watering I believe.

[DD.]

I believe you're asking.

Yes it is!

[Lardman]

  Not entirely sure how that has happened as they're watered like clockwork every other day. 

[spuriousmonkey]

Some varieties are more susceptible to it than others.

Other reason could be that just certain nutrients are missing in the soil.

[Snoop]

Give them a sprinkling of wood ash or other source of calcium and water every day for a bit. Better a little and often for toms, I believe. The same goes for sweet peppers.

[Yorkie]

Not entirely sure how that has happened as they're watered like clockwork every other day. 

It's irregular levels of moisture that are the issue, not the frequency of watering.  The weather will impact on how much watering is needed on a daily basis.  So if you're in a period of high temps and fairly breezy, the soil will be drier quicker than if it's chucking it down.

[Lardman]

I should have specified - they're in the bottom greenhouse in 12l pots.

Im floundering about in the bottom GH - Im not sure if it's the GH position, the compost, the feed or me. I also have a feeling those pots have water retaining crystal in, the first year I've tried those too.  It's probably all a bit too much new and not enough familiar for me to get things right. 

I've picked off the offending fruit (most of it), I'll see if I can find the drip watering kit I bought a couple of years back.

[Snoop]

If they're in pots in a greenhouse, then Yorkie's right. They'll be drying out in between waterings. A drip watering kit should suit them very well or water more often to ensure they stay moist (not too wet).

[Headgardener22]

BER is caused by calcium deficiency. Uneven watering is usually the main culprit but it could be over-feeding with nitrogen fertiliser or cold soil when the fruit first set all of which prevent calcium being absorbed by the plant. Some varieties are more prone to it than others, I lost all my Rio Grande to BER last year, the only one to suffer from it.

 If they're in buckets, then I agree that the problem could be the compost drying out.

I've found I'm less likely to get BER in tomatoes grown in the greenhouse borders.