The only reason this happens in mine is that the tendrils wrap around the stems especially those coming up from below, twisting and pulling them out of shape and often choking the emerging flower. I cut the excess tendrils off when I dead head which is most evenings and find this stops distortion of the stems.

Thanks for your answer but I also cut the tendrils off most days so it's not this that is causing the problem how strange.

Are they getting enough light?

I'm glad someone is actually getting some stems that are flowering, crooked or not. Wish mine were.
I've found the wind can also affect the stems if the plants aren't tied in enough and they wobble in the breeze.

Is the bed anywhere near a lawn that might have had a dose of weed and feed, Sjw?

The bed that the sweet peas are in,runs alongside the lawn. Come to think the lawn was weeded and feeded about six weeks ago and again last weekend. The ones that are starting to flower self seeded from last years plants, the flower stems are strong and long that is the ones that are not crooked. I have planted young plants at the beginning of may and they have hardly grown. Next year I might try planting seeds out in autumn, seems to produce stronger plants. I have always struggled to get long flowering stems even though I have bought more expensive plants.

I think if they had a dose of weed killer you would have more than just crooked stems.

True, Drc, much of the new growth would also be somewhat twisted, but I was thinking that a cut flower enthusiast might notice the flower stems first.
For longer, stronger stems, try feeding them seaweed once or twice a year, to bump up the potash content. Also start with one of the florist's varieties, such as 'Bouquet', or 'High Scent'.

Just been watching gardeners world live, the sweet peas were just marvellous, are they grown indoors, every bloom is perfect with such large flowers. Thanks everyone will try to obtain the varieties that you mentioned.