I thought I had solved the problem, but not quite. I can get the Mojave vpnd to work fine on Catalina *IF* I connect from my local network which is not much use. If I connect from the Internet I get 7 incoming calls, and then 7 hangups. I assumed there might have been a port forwarding problem, so I tried forwarding to the Mojave Parallels client I had been using and that worked fine. If I forwarded back to Catalina... NG. VPN Log extracts follow.