Are they the all female ones?
I haven't grown any this year as the weather is so cold, when its been a cold summer before (never as wet as this!) my cues really struggled they hated the damp cold and fluctuating temperaturesI can see the embryo cues behind your flowers have you tried misting them and if its cold closing the gh at night........I blame the weather......

You need male and female flowers for Marketmore.
If the female flowers aren't developing it's because they haven't been pollinated. If insects aren't doing it then you have to hand pollinate.

We have had to 'tickle' ours and I use a birds feather (found in garden) Balcony. Or find a friendly bee and shut the door! Hope you have luck.

Thank you for your replies.

Pamg: No, I don't think this is a female variety. I did a search on the internet & it seems they have male & female flowers & therefore the females need fertilizing from the males before they will produce "offspring" (aka fruits!)

From what I've read it seems the male flowers can open up to 10 days before the female flowers. They also seem to be more "receptive" in the early part of the day. I will have to go down early one morning & play the role of a bee & see where that gets me!

I was surprised to read about how soon the male flowers open before the females as I would have thought that the flowers need to be open at the same time - obviously if male flowers open 10 days before female ones there will be an ongoing supply of males to satisfy the females. Perhaps a lot of the open flowers at present are male & I need to be a bit more patient while waiting for the "ladies"!

Just like to say that i don't know of any females who are particularly receptive in the early part of the day.
Come to think of it, afternoons or evenings too.

I have no idea what 'make' of cucumber I have but I have only one cucumber plant in my greenhouse and can't keep up with the fruit it's producing! I had read that if the plant has both male and female flowers, the males should be removed because if you allow the female flowers to be pollinated the resulting fruit will be bitter. (The female flowers are the ones with the little fruit just behind it). So, I removed the male flowers just as they were appearing and left the female ones in place. They were watered every day in the evening - sometimes twice a day if was particularly warm. I also misted the plant once or twice a day as this is supposed to help the fruit set and helps to stop red spider mite. The cucumbers are now coming fast and thick and are very tastey with no bitterness. So, as Pamg said, I would try misting them and see if that helps.

I have never pollinated my cucumber plants either and seem to have prolific amounts of fruit - and I have not had bitter cues - have no idea which one's I grow as I get them usually from plant sales. I picked nine a few days ago and can't keep up with them!

Thats right Diggin,. They are the only cues not to give me indigestion.....

I took a small paint brush down with me this morning & tried the idea of transferring pollen from the male to the female - didn't work as there doesn't seem to be any pollen in the male flowers! I tried with 4 or 5 males flowers, as easy to id as the females, but the brush never came away with any pollen! I suppose it is yellow, isn't it? Anyway I checked the tip of the brush several times but never saw - or felt - anything on it!

Several bees entered the greenhouse while I was inside but apart from buzzing around the plants I didn't see then go into any flower. I decided to leave them to get on with the job!

There are plenty of flowers of both sexes but no more fruit seems to be forming.

Try removing a ripe looking male flower and remove the petals and pollinate by hand.
Some years ago I tried growing melons and had similar trouble, the male and female flowers seemed to come at different times, in the end I only got a couple of small ones.....wonder if its the weather......

Thanks, Pamg, like yesterday I tried pollinating with a paintbrush, transferring "invisible" pollen from male flowers to female ones. I tried doing that from flowers at different heights but I never saw anything on the the tip of the brush. I've no idea if there will be any fruits in the next few days.