Thank you mixture and Saab Dastard.
The IP address I used was a proxy one. Mine is not listed in Spamhaus so I am none the wiser. This present address allows me to communicate with Pprune but not, for example, to initiate a forum search or to send private messages. If I want to do that I have to go to the Apple shop, for example, and use their internet connection on my laptop.
Strange to say but this is entirely beyond my comprhension.