Butternut - Yellowing Leaves

Dear All,

Can someone please help me with my butternut squash? The vine is growing well and producing nice new green leaves as well as male and female flowers. However, after a while they are fading to a golden yellow colour. They then die from the edges to the centre.

They are watered regularly and have been fed a couple of times with a soluble fertilizer. The seed came from Wilco and the packet says 'Winter' F1 Hybrid.

Images attached.

Thanks in advance for any help you can offer.

Yes, that will be easy. You can just see the outline of the frame that they were under when they first went out in the picture. I'll put the plastic back over them and see what happens.