Cucumber plant not looking too good - please help!

The leaves of my cucumber plant have started to go white-ish yellow and holes are appearing in the leaves. Does anyone know what the cause might be and also the remedy? Thanks.

(Cucumber Patio Snacker F1 Plants. Grown in large planter, 5 plants which are well spaced. Lots of sunlight and regularly watered)

 

Personally i prefer outdoor ridge cucumbers (after peeling) to the under glass varieties.
I was expecting powdery mildew on yours but the photos say different..
You are doing the right things and the compost looks nice and dark. So i wonder if you are feeding the plants enough, or if possibly over feeding.
Mine are doing well and i feed them with tomato feed strictly as the label tells me.
...I made the mistake of overfeeding my runner beans many years ago, and the ground became almost toxic and the leaves shrivelled up.

 

I think it might be scorch, wet leaves in strong sunlight.
Cues like warm shady damp conditions, bit like your average tropical rainforest, but under the trees.

 

I have two in a plastic Blow-away house,one is looking good,the other is dying. This is the first year I have grown them in the full sun. Shady & damp next season,thanks pete.

 

That makes sense because my cuties are in a part shaded area which gets full sun for only 3 hours per day and i always water at the base, never on the leaves :-)