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Can anyone advise where i am going wrong please, I have 2 different varieties of squash plants that have grown and flowered at a fantastic rate in the polytunnel, however once they set fruit the fruit quickly starts to rot where the flower was attached and then they soon fall off, this includes those with no ground contact. Some of the leaves are dying back going yellow too, at the same time the plants continue to grow at the rate of knots.
They are in manured soil that also has had a small amount of calcified seaweed added - container grown.
thank you

Pollination - or rather the lack of it - is the most likely culprit.

Can bees and hover flies get in to your polytunnel?

Mike

Yes, I agree with Mike, the female flowers were not pollinated and therefore didn't continue to grow.
I've been growing squash in tunnels for several years and you most definitely have to hand pollinate to get any quantity of fruit. It's easy done, just pick a male flower and transfer the pollen to each female flower ... job done.
Provided you have male and female flowers in bloom at the same time, you should have every fruit ripen.

The other thing to be careful of is that, even though they're incredibly thirsty plants, they hate getting their fruit/flowers wet. Only water (without rose if you're doing it by hand) around the roots.

As the other guys and girls have said, lack of pollination is your most likely cause. But it could also be calcium deficiency, water once a week with 1 tsp epsom salts in 1 gallon water and mulch with old grass clippings (In between your regular watering). The condition is called blossom end rot.

Thanks for the replies, the doors or at least one door is open on sunny days but sounds as though pollination or lack of could be part of the problem then - will have ago at pollinating the flowers tomorrow. Am keeping the flowers/fruits as dry as I can.
Added the sea cal a couple of days ago but hopefully that should provide some slow release calcium. A bit puzzled about the epsom salts as I thought these were Magnesium sulphate, but maybe magnesium is also needed. Will do some mulching and also add broken egg shells I think.
Is the leaf yellowing indicative of the Calcium deficiency?
Hopefully will have some better luck with growing squashes now - thank you all!

Yellow leaves can be a sign of Magnesium deficiency.

Thanks Green Aura, have some epsom salts in so will add some to the water then