Tiny cucumbers

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Bob, twenty cucs showing at one time is a heck of a lot for the plant to support. In my experience they don't usually come on quite so fast - usually there are a couple of biggish ones, two or so intermediate and a couple just forming.
You don't say what variety you are growing - some varieties are quite small, but 40mm is a bit stunted.
My best guess, and is is only a guess is that you've got the feeding wrong and thye plant has made too much green growth at the expense of developing good sized fruits. A possible remedy might be to remove at least half the crop and then feed regularly with something like Tomorite which is a high K feed.

 

These are just a couple of pics of my cues the 1st 8" long and the same around and i have several nearly as big, they are from Beith Alpha seeds i bought back from Cyprus, unfortunately they have male flowers,but it doesnt take 2/3 mins to check them out

 

Bob, try feeding twice a week and take off a half doz near the base.
I've just had another look at the photo you posted and the cuc looks very like a prickly ridge cucumber and these don't grow very long, though they will get quite plump if left.

 

Are these Ridge / Outdoor cucumbers grown indoors? I don't know if there are any issues with that? But if so they will need pollinating.

 

Bob, I'm pretty sure that the plant is a ridge type, maybe Marketmore. It looks like the plant has reached the top of your greenhouse so I'd let the sideshoots develop - you can train them along wire or string if they get too long.
You clearly don't need to pollinate by hand, given the amount of fruit that has been set unaided. I've grown ridge cucs in a polytunnel and greenhouse for the last 4 or 5 years and have never had to hand pollinate.

 

Possibly some magnesium deficiency there? I don't know if that would cause fruit ripening to be delayed / suppressed?

 

Harvested the large cucumber, it was the first to grow on the plant , it tasted lovely ,but, as i was slicing it the centre seed area was develpoing a "split" and it finished being quite wide it was like a hollow in the middle,

did i leave it to long on the plant , i have watered every day ,twice now the plants are bigger and bearing fruits ,morning and evening,and it has been fed (tomato food) twice weekly

 

Could I also ask if it's a good idea to remove some of the leaves to promote cucumber growth?