FOUND IT!
many thanks for suggestions. might be useful for others. The wordpress site was trying to access an unused (not needed) certificate for the server root.
Cut a long, long story short, In the ServerAlias lists in the virtualhost file had “lost” or I had typed wrong, the ServerName directive and only had all the ServerAlias directives, so (can’t find any documentation) if the ServerName directive is missing, then looks to the root FIRST. Our root was server.watchet.net which displayed holding page but adding https://server.watchet.net did not but jumped to our faulty page https://themarinerwatchet.co.uk
Great learning curve, but this migh tbe useful to others
John
UK