Hi Mick,
I'm so sorry for the delay on this one....
We say you should be there at least an hour before departure - that will give you time to get through the check-in process, pick up a magazine (if you wish) and take a stroll up to the platform.
The return time in the system is 16:03hrs (as always this is provisional, subject to change and would be confirmed in your final documents)
It is uncommon for timings to change once the final documents have been issued (as we tend to post them out so close to departure). That said, we are at the mercy or Eurostar and other suppliers, so if they were to make a last minute change, a member of our team would be in touch to notify you of this.
Kindest regards,
Julia