I’m coming from heathrow at 13:15 and will probably book the 16:23 (the next one isn’t until 17:48) train for Paris. What if customs is a b*tch and it takes me an hour extra there for some reason and I arrive at the Waterloo station as my 16:23 Eurostar is off to France.

What happens then?

Also, I’ll buy the ticket before I leave for Europe but what about seat reservations. Do I need one for that train?

thanks
Jason