why have my cucumber plants given a great crop and now turning yellow

ours were a bit like this too - not a steady supply as it were, they seem to go a bit mad, then look exhausted. In our case it could have been due to irregular watering? Obviously they are thirsty, hungry plants as they are working hard, but they can't be doing with overwatering by a guilty gardener (me)

I would think the plantsare dying after giving thier all.
It may be Red or Two spotted mite if the leaves are a yellowy green colour. They live on the underside of the leaves in dry conditions and feed on the plant, They are tiny spider like mites that can easily kill plants in a greenhouse.
Humid atmosphere will help get rid of them (not completely), but it sounds like its too late.
However, too humid an atmosphere will encourage mildew- another bad thing in a greenhouse!

Over watering may be a cause too as well as to hotter conditions