Just a simple thanks to the community for their advice in their posts. They have helped no end through a difficult challenge today.
Regrettably we had to change next weeks plans from a 5 travel day cross-europe jaunt to a quick dash to Monaco and home (flight and hotel already paid for).
Our return home will now be Nice - Lille - London - Leeds, needing those dreaded reservations for TGVs and Eurostar.
Because of the advice I managed to book 2 TGVs (1 at 10 euro and 1 at 20 euro) and an early afternoon Eurostar through B-Europe (link from posts) with Pass Cover Numbers (from a link in a post). At several points it looked hopeless with error messages and starting again but we stayed with it thanks to the advice in many posts on regular work rounds.
The weirdest was on the b-europe site where it would not accept mouse clicks for selecting entries on drop down lists but would let me select with my touchscreen. That took some time (and a few choice words) to sort.
We even managed to get all 3 as e-reservations within seconds.
So my key tip is if you get an error message that a transaction has failed, be paatient, start again and check all your entries.