@Eurostar All sorted. Simon in the ticket office was incredibly helpful :) don't know why the machine wouldn't work for us but glad to have it sorted and be on a later train this afternoon.
@Eurostar were supposed to be on the cancelled 12.24 St Pan to Paris. For some reason can't change tickets online and machine showing no seats available on other trains today. In line for ticket office now but can you advise if there are still seats available to Paris today?