Bikes on Eurostar?

I looked into this quite a bit over summer to get me and 5 mates and our bikes back from Paris after a mini tour...

Nowadays each train only has 2 (fully assembled) bike slots plus a handful more for bikes in boxes. You are unlikely to get away with bagging it up.

All being well process goes like this:

1. call eurostar dispatch and check which trains still have bike slots
   
2. once you know there's a conveniently timed train which has spaces for bikes book your passenger ticket online to coincide with the train with space available
   
3. call back eurostar dispatch with your booking number and pay an extra fee to put the bike on the train.
   
   

If the train you are booked on doesn't have any full assembled bike slots, they also have slots available for a bike in a box (they will provide the box and tools to take it apart when you drop off). I can't recall the exact number but I think it was something silly like only 6 spaces available for boxed up bikes.

Alternative Option (which is now my preferred option)

You can drop off your bike at eurostar dispatch at any time only showing them you're passenger booking and they will deliver your bike to whichever destination your heading within 24 hours of drop off and keep it there for you for up to 14 days. This option was really helpful for us as we arrived in Paris on the Friday, dropped off our bikes that afternoon. Got the Eurostar back to Kings Cross the following Sunday where our bikes were there waiting for us.

This cost approx £25 on top of the original booking and you can pay when you drop off.