Just to end the thread I started, I finally replaced the 8 plugs last night, taking about two and a half hours. I wanted to take my time and did each plug seperately before going on to the next. The four at the back were certainly more awkward but doable. I bought the Carlube copper grease which made the new plugs screw in much easier.. Thanks Culzean for that tip. I used it sparingly and should have enough in the tube to change the plugs a few more hundred times!
I am not altogether certain if the plugs needed changing. The four back ones looked to be in the same condition as the four front ones, all of them having that healthy dry brown deposit on them. I can tell you that when I 'guaged' them, it took an 80+40 in the gap, compared to an 80+20 on the new ones!
My daughter told me today that she thought it was driving really well and seemed to have more "oomph", as she described it.
As an aside, I noticed that the front plate on the Jazz, states that it is assembled in China, but as someone else mentioned on this site, Honda would have high quality control in whatever country it is made. I know that everything works well on this motor. I even tried the air-conditioning on my test drive after the plug change. While slightly noisy, it seemed to work but was hard to judge, due to the present cold climate. I will try again once the warm weather returns.
Anyway, folks thanks again for all your valued input. It gave me great peace of mind to carry out the spark plug operation.