Hi Buzztone,
Many thanks for your response.
I do think GD and FreeType PHP are installed (I am the administrator). I believe this because I have seen the codes on the form (albeit briefly) and the images are being generated in the wp-content/uploads/wpcf7_captcha folder.
However, the steps in that first link you sent make absolutely no sense to me – still learning! The same is true for the other links I have found. I probably need a more basic explanation!