My cucumbers in the greenhouse have exactly the same problem as your top picture. At first I thought it was something called downy mildew and I sprayed it but I found some webs on one of the leaves and I think it is red spider mite. The problem is spider mites like dry conditions but cucumber plants don't like it too damp or they get powdery mildew - seems you can't win. If anyone else knows what it is I would be glad of an answer. Looking at your top pic I can see a web at the join of the leaf with a stem so perhaps it has red spider mite too.

Two things ccome to mind......lack of nutrients and leaf miner. I havn't seen the second one before but it looks like something is burrowing around inside the leaf?
Can't be sure tho.

The first one is spider mite. The best remedy is wettable sulfur dust, but that is best applied when the temperatures are not going to go above 29.5º C in the next 24 hours. Otherwise, vigorously hose the plants off in the morning, and leave the ventilators open for several hours after, to let the leaves dry, and the humidity drop.

The second one looks like "mesophyll collapse", which is caused by letting the plants wilt severely between soakings.

Ah thats interesting...they do look a bit floppy. Not heard of that before.(the second one!)

That's kind of the terminal stage of wilting, where the spongy cells in the middle of the leaf structure collapse and die--often crushed by the weight of the wilted leaf, and their own lack of water pressure. The cells nearest the veins are often spared, both supported by the vein fibers, and better supplied from limited water sources.

So basically that leaf has had it then? And the plant?

The leaf will probably hang on, though it is less efficient now, and the plant will be using some of its energy to recover and grow new leaves, instead of producing cucumbers.

I shall make sure I water mine when they need it and not let them get to that stage! Thanks.