I have now had an email response which effectively suggests the problem that I experienced in not being able to book via Amtrak.com was an input error/omission by myself.
I know this was not the case as I know for sure that my successful booking on the Amtrak App used exactly the same entries as I had been trying on Amtrak.com (and v v's similar booking experience to mine would also seem to indicate that it was not input errors).
I have responded to Amtrak Customer Support updating them that I had succeeded using the same inputs but on the Amtrak App and also that I was aware that another European "booker" had experienced exactly the same problem (and hypothesised that perhaps the website is only failing non North America bookers).
I will update if I get any further response.
I was trying to book 3 separate journeys that were not directly connected, tried a number of times to make the bookings online over several days with several computers but the same problems persisted. Please feel free to use my examples if you wish. But as you when you used a different booking method to their online pages, it was easy once we phoned reservations.
Unless customers explain to them what is going wrong they wont know, good luck.