Correct temperature for a greenhouse...

40 degrees is way too high. Open up the doors to let air in and see if you have any old sheets that you can drape over the green house to give some shade.
10 to 20 degrees will get most things growing nicely. Most seeds start to germinate from 5 to 15 degrees, but some seeds need a little more heat.
Melons, cucumbers and tomatoes like temperatures of over 20 degrees to germinate but once growth has started they will carry on growing at 15 degrees. Later on they will need temperatures of over 20 degrees to grow fruit well. Less than 20 degrees and tomatoes will stop forming fruit. Over 25 degrees and a lot of other plants start to suffer from excessive heat and need to be watered more often or they will stop growing.
My greenhouse is set so that the auto vents will open between 20 and 25 degrees. When the seedlings are out and I am only growing the plants that like warmer temperatures I will alter the vents just a little to keep it around 23 degrees before the vents open. I will also have blinds drawn to stop the green house over heating.
You can cook the plants in a greenhouse very easily if there is no shade cover or ventilation.
Jax

Thanks for the advice. My greenhouse is made of semi transparent sheeting - you can't really see through it, so not sure I need to add shading, but I am going to add verntilation as suggested by Andrewo in another post , so that sould help the problem. I'll keep an eye on the thermometer and try and keep it between 10-20 degrees.