Richard wrote:

- do I have a ticket for the whole journey - might help if things go
wrong?

Yes- whatever you do make sure you have a Eurostar ticket in advance.
Last year I went from Gard du Nord from Gare de Lyon to catch the last
London train and got there with an hour to spare but they had no
dedicated counter to sell economy class tickets for that train and
several of us had to queue behind people buying complicated advanced
stuff. Eventually, about five fo us were forced to buy business class
tickets (over 300 euros) as there were of course about three people
sitting at those desks doing nothing. I emailed Eurostar to complain
later and they did send me a free open first class return.

However, I do realise that even a single economy Eurostar fare can be
costly - much more often than a return.

E.