Aargh... Eurotunnel booking

(perhaps a bit off-topic here)

I'm trying to do an online booking for a Eurotunnel journey. First you have
to work yourself through three pages and fill in endless forms. At the end
you have already selected a fare, the departure times (chosen by the web
site itself, based on your travel wishes), I click "book now", and I get:
---
No more booking availability, choose another departure date and time 
[U0010].
REQUESTED MISSION FULL IN TOTAL 
---

So what does this mean? Those trains are full? If so, why did they suggest
these departure times in the first place?

What's worse, you return to the first page and have to go through the
entire process again. I tried three times now and I'm getting fed up with
this.

Don't they understand that if they want to have customers, they need to
make it a little easier for them?

Regards,
David