5 out of 6 Cukes dead and crispy.

I reckon you could resow.  They grow so fast I'm sure you would get a crop this summer 

Could be the air in the greenhouse is too dry.  They like humidity, so maybe stand the pots on a tray of gravel that you could have some water standing in   

I suppose it could be humidity, although with only 1 leaf I wouldn't have thought it would have made that much difference?  I'll grab some pictures tomorrow, but to be honest everything in the greenhouse is looking worse for wear this year. 

It is certainly not too late - one year I sowed some at the beginning of July and still did okay. As New Shoot said they need humidity but they also need to be shaded from the sun. The way my father showed me to do this was to make a sort of umbrella out of newspaper and tie it to the plants' supports just above the plant. As the cuke grows taller the umbrella is pushed further up the support. Believe it or not it really does help to protect the plants.

Just as well I cannot post pics of my cukes - they are looking really healthy - you might never speak to me again.      Should be picking the first later this week - although that is a bit later than normal.

I use thin fleece pegged to the glass permanently. 

That's the normal 17g fleece, not the thicker 30g fleece.  Seems to be working.