Hi Syed,
Thanks for your answer.
I'm using the built in Windows feature to [Extract all files]. I've tried the steps you suggest with the following results:
Step 1 - I can unblock the zip file, but this does not unblock the individual files once I've extracted them.
Step 2 - The individual folders do not have an Unblock button - just the files within them, so Step 2 does not apply to my files.
Step 3 - If I select any individual file, I can open its Properties and Unblock it; however, if I select all the files in a folder, the Unblock button does not show - so I cannot unblock them all at once in this way.
I would be very grateful if you have any further suggestions to help me.