Kay wrote:
First thought - are you getting any female flowers yet? They tend to
produce male flowers first, and later female flowers which have a
visible embryonic squash at the base.

That's probably it. I didn't know the difference. They look like male
flowers (according to Google)

Your soil looks a bit dry and lacking in humus - I sympathise with you
having to cope with that!

It's heavy clay. I dug in some manure to most of the veg plot over the
winter, but left that bit because there was some dried foliage that looked
like it might sprout (it's my first year here). It only sprouted weeds, so
I dug it up and planted the squashes there, but had no more manure left.

I've sunk a pot in to help watering as it was just running off the surface,
which should help. Some of the undug ground is like cement in this weather
though

Thanks
Theo