It is saying that the FOLDER where you put the catalog file is unwriteable.
You must change permissions on the folder so that you have WRITE permission, or move the catalog file to some folder where you have WRITE permission.
Thank you for your reply. I found it rather daunting as I have no idea how to change WRITE permission on a folder. However, after reading your message I went back to my laptop and this time the catalogue opened immediately without any messages of file being unwritable appearing - despite my having done nothing different. This is typical Lightroom in my experience - wonderfully intuitive for developing photos but whether the catalogue opens or not seems to depend on the phase of the moon. This has caused me more stress in the past than anything.
Anyway the problem now is the catalogue folder I want to develop displays the correct number of images in the Library but they are all blank with ? in top right corner of each photo. When I click on the ? it says original image file cannot be located. But all the original image files and associated xmp attachments have all been copied on to the laptop too. Have you any idea of why they cannot be located and is there a simple method of making the catalogue folder locate them. Their are 840 images in this folder so I don't want to have to go through the process of having to locate each one individually. I would be grateful for any help