Gents. Thanks for all your posts on this subject. I have made progress and decided the best option was that posted by Silversidelines. So, I have reduced slightly the diameter of the nylon centres and, additionally, reduced their thickness, including shaving off the front,central, boss. As things stand they appear to lie flat. However, Malta's DIYers are not able to purchase Evostik Pipeweld so, at present, I have used UHU.
Lets hope the matter is solved.
Jim